I wanted to stay out of tiering discussions as much as possible, but since haxiom wrote up such a beautiful post I felt like I should too. Like haxiom I feel that Mega Gengar should be innocent until proven guilty, and looking through the arguments raised I can't judge Mega Gengar as guilty. I simply don't find the arguments for banning anywhere near convincing.
Pro-ban
There are a few other pro-ban arguments that I've seen not addressed by haxiom; I'll put those at the end.
Gengarite should be banned because it has a great combination of speed, power, Taunt, Destiny Bond, and Perish Song in addition to Shadow Tag which makes it uncompetitive.
I find this reason silly because under the fancy wording it's an argument for banning Mega Gengar because it's too strong. It's already been stated that Arceus and Xerneas (two of the biggest threats in the metagame, arguably even bigger than Mega Gengar) would not be suspected. Banning Mega Gengar because it's too strong goes against the fact that Arceus and Xerneas won't be suspected.
Gengarite should be banned because it forces an overly matchup based metagame which is inherently uncompetitive.
It's already the case that games are matchup-dependent. Here're a couple of quotes:
By Edgar, who as far as I know is pro-ban: "When teambuilding for Ubers, don't try to directly counter every single Offensive/Defensive threat, I have yet to see a team that it is able to do that, in other words, it is nearly impossible to achieve, especially in a generation like XY, where team matchup decides a lot of the battles, the best you can do is build a team that can do well against standard builds and hope that your opponent doesn't bring that threat you're weak to."
By Donkey in his RMT, commenting about his Sticky Web team, my emphasis: "The Second Game: http://replay.pokemonshowdown.com/smogtours-ubers-1521
Problems using the polished Shuckle version. He had a significant matchup advantage, which is part of Sticky Web's niche. Also, he is the one who thought of using Mental Herb."
I don't see how Mega Gengar makes games even more matchup-dependent.
Gengarite should be banned because it takes away meaningful choice by preventing the opponent from switching.
There are a few of things going against this reason. The first is that often one does not actually have a choice, for example if you have CM Kyogre in against a Scarf Zekrom, you kinda have to switch. You don't have to, but if you want to win (and still have a Zekrom check available) you still kind of have to. The second thing is that it's an argument against banning Shadow Tag, not Gengarite. We're suspecting Gengarite, not Shadow Tag. Finally, Shadow Tag has existed since Gen 3. If it wasn't banned then because of taking away meaningful choice in previous generations, why should it be banned now?
Gengarite should be banned because it naturally forces a lot of 50/50s in the current metagame, which is unhealthy and uncompetitive.
I'll address this together with the "Gengarite should not be banned because it encourages high level prediction and double-switching" pro-ban argument. haxiom feels that Dice hit this one pretty hard; I do not. The point is that 50/50s are coin flips only if both players play completely randomly. Humans aren't random. It's possible to read the other player and predict what he'll do - and in the end that's largely what Pokemon is about, isn't it? I'll cite another example as well. Rock-Paper-Scissors is a perfectly balanced game in which it's impossible to gain an advantage over a computer who chooses its weapon as random. Why, then do Rock-Paper-Scissors tournaments exist? These tournaments work because humans are not random.
So while Dice may not think so, I'll say I think that the doubling Mega Gengar causes is a skill.
Gengarite should be banned because it limits teambuilding to a point where most fairies and grass types are almost unviable, and is therefore uncompetitive.
Implying we don't see Fairy or Grass types, but Xerneas still dominates the usage charts and Sylveon still got an analysis. Even in the two stall teams that Donkey posted, there are two Fairies and two Grass types. Grass types are rarer, but it seems to me that Ho-Oh is a bigger factor in why they're uncommon than Mega Gengar. I'm not convinced. Besides Stealth Rock limits teambuilding to a point where the Rock-weak types are severely hindered (especially those 4x weak to SR) and I don't see anyone calling for SR to be banned.
But Gothitelle and Wobbuffett ...
The problem with this is that it's Gengarite that's being suspected, not Shadow Tag. Any arguments based on Gothitelle and Wobbuffett are irrelevant here.
Gengarite should be banned because it often gets more than one KO a match.
I think this argument is naive. Deoxys-S is unlikely to get any KOs, yet it's a perfectly viable Pokemon. StallTwo isn't likely to get many KOs, which doesn't make it unviable. It's a team effort and the winner is the one who faints all his opponent's Pokemon.
Anti-ban
I think there are some arguments against banning that wasn't addressed by haxiom, which I'll put at the end of the post.
Gengarite should not be banned because it encourages high level prediction and double-switching.
Addressed above.
Gengarite should not be banned because this is Ubers, and we do NOT ban stuff in Ubers.
This argument is obviously flawed because we do ban stuff in Ubers. Sleep clause, OHKO clause, even Prankster clause exist. However we can amend it to be more meaningful: we do not ban Pokemon in Ubers. A Gengarite ban is essentially banning a Pokemon. Can you imagine a Pokemon that's too uber to be uber? What about if someone's favourite Pokemon is Mega Gengar, and he can't use it anywhere? I personally find both scenarios rather distasteful.
Some people might ask what happens then if there ever is a Pokemon whose only ability is Moody - do we ban or not? I'll say we cross the bridge when we get there. We're not there yet, and hopefully we'll never be, but if it ever comes to pass that's when we decide what to do.
Gengarite should not be banned because it is to fragile to do anything useful.
This argument is silly because Deoxys-A is even more fragile and it's still a legitimate threat in the tier.
Gengarite should not be banned because it takes one turn to mega-evolve, allowing opportunities to revenge or pursuit trap it.
As haxiom said Pursuit trapping Mega Gengar is not a guaranteed business, but nonetheless this one turn taken to mega evolve is a big deal. It means you have one get-out-of-jail-free card; you can switch to a Pokemon like Kyogre that can sponge Mega Gengar without problems and threaten to sweep. It means if the other guy still wants to trap your Grass Arceus / Sylveon / whatever, he has to outplay you one more time. With team preview giving players advance warning of an opposing Mega Gengar, I feel like one can't complain about a Mega Gengar popping up out of nowhere and trapping one's Grass Arceus (or similar).
Gengarite should not be banned because, like any other Pokemon, it is beat in the teambuilder.
I think this argument is a good one. Most of the Pokemon that are trapped by Mega Gengar have some way to beat it. There's a reason why Grass Arceus's analysis page has a slash for Earthquake (curiously, Stone Edge is slashed first - apparently QC thinks Ho-Oh is a bigger threat than Mega Gengar, and yet here we are suspecting Mega Gengar instead of Ho-Oh). Even those that cannot outright kill Mega Gengar can usually equip Shed Shell and run. Yes there's a rather big cost associated with not having Leftovers or whatever, but the point remains that if you want to escape from Mega Gengar, you usually can. It's just a question of, do you want to?
Gengarite takes skill to use.
I think this is a legitimate reason. It's not hard to say "just switch Mega Gengar in on your opponent's Blissey", but you still can't do that willy-nilly because you need to find the space to mega evolve, you can be double switched out of, you need to be able to play around your opponents' team until you can force the Blissey out and trap it, and so on. This isn't like Moody or Prankster Swagger. With Prankster Swagger one can almost write a simple program on how to play: if the other Pokemon is not immune to Thunder Wave or already paralyzed, use Thunder Wave. Else if the other Pokemon is not already confused, use Swagger. Else if you are not already behind a Substitute, use Substitute. Else, attack. It works against almost everything (there are exceptions such as an opponent that's managed to Substitute and Own Tempo Pokemon, but you get the idea). Same goes for Moody, and arguably evasion as well since one can rely on Double Team / Minimize working against almost every Pokemon, but not Mega Gengar.
Gengarite takes up mega evolve slot.
No escaping from this one. Using Mega Gengar means you don't get to use other mega Pokemon, and some of the other mega Pokemon are very strong: Mega Blaziken, MMX and MMY, and Mega Scizor spring to mind.
Finally there are a couple of questions I'd like to ask pro-ban players, because I've not seen them answered.
1) If Mega Gengar is so good, why is it that many other Pokemon are more common? Last I checked, Groudon, Xerneas, Arceus and Kyogre were all more common than Mega Gengar, even at the top end of the ladder. I did not look at every replay, but in the finals and semifinals of the SPL there were 6 Arceuses (every team had one!), 4 Yveltals and only 3 Mega Gengars. Why?
2) If Shadow Tag is so uncompetitive, why is it that it wasn't banned in previous generations? It's not like trapping strategies didn't exist, I vaguely remember a Sub + Hone Claws Dugtrio strategy meant to trap Pokemon that spam Thunder in previous generations. Wobbuffett's ability to trap Choice Scarf Pokemon and open up a sweep has been known since at least Gen 4. One cannot switch out of it, it takes away meaningful choice, etc, and yet it's never been banned. Why?
Pro-ban
There are a few other pro-ban arguments that I've seen not addressed by haxiom; I'll put those at the end.
Gengarite should be banned because it has a great combination of speed, power, Taunt, Destiny Bond, and Perish Song in addition to Shadow Tag which makes it uncompetitive.
I find this reason silly because under the fancy wording it's an argument for banning Mega Gengar because it's too strong. It's already been stated that Arceus and Xerneas (two of the biggest threats in the metagame, arguably even bigger than Mega Gengar) would not be suspected. Banning Mega Gengar because it's too strong goes against the fact that Arceus and Xerneas won't be suspected.
Gengarite should be banned because it forces an overly matchup based metagame which is inherently uncompetitive.
It's already the case that games are matchup-dependent. Here're a couple of quotes:
By Edgar, who as far as I know is pro-ban: "When teambuilding for Ubers, don't try to directly counter every single Offensive/Defensive threat, I have yet to see a team that it is able to do that, in other words, it is nearly impossible to achieve, especially in a generation like XY, where team matchup decides a lot of the battles, the best you can do is build a team that can do well against standard builds and hope that your opponent doesn't bring that threat you're weak to."
By Donkey in his RMT, commenting about his Sticky Web team, my emphasis: "The Second Game: http://replay.pokemonshowdown.com/smogtours-ubers-1521
Problems using the polished Shuckle version. He had a significant matchup advantage, which is part of Sticky Web's niche. Also, he is the one who thought of using Mental Herb."
I don't see how Mega Gengar makes games even more matchup-dependent.
Gengarite should be banned because it takes away meaningful choice by preventing the opponent from switching.
There are a few of things going against this reason. The first is that often one does not actually have a choice, for example if you have CM Kyogre in against a Scarf Zekrom, you kinda have to switch. You don't have to, but if you want to win (and still have a Zekrom check available) you still kind of have to. The second thing is that it's an argument against banning Shadow Tag, not Gengarite. We're suspecting Gengarite, not Shadow Tag. Finally, Shadow Tag has existed since Gen 3. If it wasn't banned then because of taking away meaningful choice in previous generations, why should it be banned now?
Gengarite should be banned because it naturally forces a lot of 50/50s in the current metagame, which is unhealthy and uncompetitive.
I'll address this together with the "Gengarite should not be banned because it encourages high level prediction and double-switching" pro-ban argument. haxiom feels that Dice hit this one pretty hard; I do not. The point is that 50/50s are coin flips only if both players play completely randomly. Humans aren't random. It's possible to read the other player and predict what he'll do - and in the end that's largely what Pokemon is about, isn't it? I'll cite another example as well. Rock-Paper-Scissors is a perfectly balanced game in which it's impossible to gain an advantage over a computer who chooses its weapon as random. Why, then do Rock-Paper-Scissors tournaments exist? These tournaments work because humans are not random.
So while Dice may not think so, I'll say I think that the doubling Mega Gengar causes is a skill.
Gengarite should be banned because it limits teambuilding to a point where most fairies and grass types are almost unviable, and is therefore uncompetitive.
Implying we don't see Fairy or Grass types, but Xerneas still dominates the usage charts and Sylveon still got an analysis. Even in the two stall teams that Donkey posted, there are two Fairies and two Grass types. Grass types are rarer, but it seems to me that Ho-Oh is a bigger factor in why they're uncommon than Mega Gengar. I'm not convinced. Besides Stealth Rock limits teambuilding to a point where the Rock-weak types are severely hindered (especially those 4x weak to SR) and I don't see anyone calling for SR to be banned.
But Gothitelle and Wobbuffett ...
The problem with this is that it's Gengarite that's being suspected, not Shadow Tag. Any arguments based on Gothitelle and Wobbuffett are irrelevant here.
Gengarite should be banned because it often gets more than one KO a match.
I think this argument is naive. Deoxys-S is unlikely to get any KOs, yet it's a perfectly viable Pokemon. StallTwo isn't likely to get many KOs, which doesn't make it unviable. It's a team effort and the winner is the one who faints all his opponent's Pokemon.
Anti-ban
I think there are some arguments against banning that wasn't addressed by haxiom, which I'll put at the end of the post.
Gengarite should not be banned because it encourages high level prediction and double-switching.
Addressed above.
Gengarite should not be banned because this is Ubers, and we do NOT ban stuff in Ubers.
This argument is obviously flawed because we do ban stuff in Ubers. Sleep clause, OHKO clause, even Prankster clause exist. However we can amend it to be more meaningful: we do not ban Pokemon in Ubers. A Gengarite ban is essentially banning a Pokemon. Can you imagine a Pokemon that's too uber to be uber? What about if someone's favourite Pokemon is Mega Gengar, and he can't use it anywhere? I personally find both scenarios rather distasteful.
Some people might ask what happens then if there ever is a Pokemon whose only ability is Moody - do we ban or not? I'll say we cross the bridge when we get there. We're not there yet, and hopefully we'll never be, but if it ever comes to pass that's when we decide what to do.
Gengarite should not be banned because it is to fragile to do anything useful.
This argument is silly because Deoxys-A is even more fragile and it's still a legitimate threat in the tier.
Gengarite should not be banned because it takes one turn to mega-evolve, allowing opportunities to revenge or pursuit trap it.
As haxiom said Pursuit trapping Mega Gengar is not a guaranteed business, but nonetheless this one turn taken to mega evolve is a big deal. It means you have one get-out-of-jail-free card; you can switch to a Pokemon like Kyogre that can sponge Mega Gengar without problems and threaten to sweep. It means if the other guy still wants to trap your Grass Arceus / Sylveon / whatever, he has to outplay you one more time. With team preview giving players advance warning of an opposing Mega Gengar, I feel like one can't complain about a Mega Gengar popping up out of nowhere and trapping one's Grass Arceus (or similar).
Gengarite should not be banned because, like any other Pokemon, it is beat in the teambuilder.
I think this argument is a good one. Most of the Pokemon that are trapped by Mega Gengar have some way to beat it. There's a reason why Grass Arceus's analysis page has a slash for Earthquake (curiously, Stone Edge is slashed first - apparently QC thinks Ho-Oh is a bigger threat than Mega Gengar, and yet here we are suspecting Mega Gengar instead of Ho-Oh). Even those that cannot outright kill Mega Gengar can usually equip Shed Shell and run. Yes there's a rather big cost associated with not having Leftovers or whatever, but the point remains that if you want to escape from Mega Gengar, you usually can. It's just a question of, do you want to?
Gengarite takes skill to use.
I think this is a legitimate reason. It's not hard to say "just switch Mega Gengar in on your opponent's Blissey", but you still can't do that willy-nilly because you need to find the space to mega evolve, you can be double switched out of, you need to be able to play around your opponents' team until you can force the Blissey out and trap it, and so on. This isn't like Moody or Prankster Swagger. With Prankster Swagger one can almost write a simple program on how to play: if the other Pokemon is not immune to Thunder Wave or already paralyzed, use Thunder Wave. Else if the other Pokemon is not already confused, use Swagger. Else if you are not already behind a Substitute, use Substitute. Else, attack. It works against almost everything (there are exceptions such as an opponent that's managed to Substitute and Own Tempo Pokemon, but you get the idea). Same goes for Moody, and arguably evasion as well since one can rely on Double Team / Minimize working against almost every Pokemon, but not Mega Gengar.
Gengarite takes up mega evolve slot.
No escaping from this one. Using Mega Gengar means you don't get to use other mega Pokemon, and some of the other mega Pokemon are very strong: Mega Blaziken, MMX and MMY, and Mega Scizor spring to mind.
Finally there are a couple of questions I'd like to ask pro-ban players, because I've not seen them answered.
1) If Mega Gengar is so good, why is it that many other Pokemon are more common? Last I checked, Groudon, Xerneas, Arceus and Kyogre were all more common than Mega Gengar, even at the top end of the ladder. I did not look at every replay, but in the finals and semifinals of the SPL there were 6 Arceuses (every team had one!), 4 Yveltals and only 3 Mega Gengars. Why?
2) If Shadow Tag is so uncompetitive, why is it that it wasn't banned in previous generations? It's not like trapping strategies didn't exist, I vaguely remember a Sub + Hone Claws Dugtrio strategy meant to trap Pokemon that spam Thunder in previous generations. Wobbuffett's ability to trap Choice Scarf Pokemon and open up a sweep has been known since at least Gen 4. One cannot switch out of it, it takes away meaningful choice, etc, and yet it's never been banned. Why?