Vote: NO BAN
Note, I may use the terms Gengarite and Mega-Gengar interchangeably. I would also like to apologize for typos.
I disagreed with a majority of the pro-ban arguments, and the burden of proving that mega-gengar was banworthy was on them. The argument that Mega-Gengar had a great combination of speed, power, and moves is something that I disliked. This argument purports that Mega-Gengar is broken, but the ubers tier is full of broken threats, so this seems invalid. I also disagree with the argument that Gengarite should be banned for forcing overly matchup based games. It is impossible to build a team that can directly counter every offensive threat, your best bet is to build a team that does well against the standard threats. Your team may have an exploitable weakness (offensive teams weak to trick room/sticky web, etc.), but matchup advantages have always existed and I do not think this criteria is sufficient for a ban. The argument about the 50/50s caused is also one I found lacking. As Minority Suspect, Shrang, and others noted, the ubers metagame is already full of 50/50s (Zekrom, for example), and guessing incorrectly could be just as devastating. For the argument about fairy/grass types becoming less viable and how teambuilding is limited, several users pointed out a counterargument that I found reasonable. The giratina formes have significantly decreased in usage both in tournaments and ladder use this generation, due to the omni-presence of the new xy legendaries. Does Yveltal/Xerneas reducing the viability of giratinas make them banworthy? This draws to a point on the anti-ban side that I support, that mega-gengar is beaten in the teambuilder. On the arceus grass analysis for example, one of the move slashes is stone edge. It has already been identified how a key threat, ho-oh, significantly threatens grassceus, and how stone edge has viability for specifically that reason. I do not see how running something like dark pulse on grassceus or a similar situation with a different mon (shed shell on blissey) is any different. Mega-Gengar is a threat that should be addressed, not ignored in hopes that it is banned. I disliked the argument about how Mega-Gengar can run moves to get around its usual pursuit trap counters. How is this any different than an e-killer running overheat to get around mega-scizor/ferro or groundceus/mmx running ice beam to smash gliscor/lando-t? Mega-gengar suffers from Bad 4MSS, it wants to focus blast/shadow ball/sludge wave/taunt/destiny bond/protect/sub/reflect type/wisp/icy wind, etc. Mega-Gengar cannot do everything, and it might easily be taken out by a pursuit trapper it was not prepared for. It may just happen that mega-gengar can beat your pursuit trapper, and I do agree with there is no way to 100% pursuit trap Mega-gengar, but I really disagree about how that is a banworthy statement when there are plenty of other cases about mons adapting their movesets to get around specific threats. Saying that something is uncounterable depending on its moveset seems like a flimsy argument, run something like av tomb or scarf-yveltal (semi-joking, but seriously wins unless Mega-Gengar d-bonds on the switch). One argument that I did not really see mentioned in the thread is how Gengarite is an opportunity cost as well, in terms of using your team's mega-evolution. There are a plethora of other users of mega-evolutions that can give your team enormous offensive presence, like the mewtwos, blaze, mega-luke, charizards, mega-mawile, etc. Opting to not use one of these in favor of Mega-Gengar is indeed an opportunity cost.On a similar vein, several arguments on the pro-ban side seemed flimsy. The turn to mega-evolve is not really that big of a deal, although is does make it more vulnerable to pursuit trapping as opposed to something like gothielle. It's fragility really doesn't matter, as there are smarter ways to bring it in to mega-evolve than on a resisted move. The argument that this is Ubers, we don't ban shit was flimsy. While I agree with the sentiment about it being a precedent, it really doesn't address any pro-ban arguments. The one argument on the pro-ban side I found interesting was about how Mega-Gengar removed choice and does not encourage high level prediction/double-switching. While I thought this argument had some solid basis, I still disagreed with it. Removing choice for 1-2 turns does not invalidate every other choice you've made in the game (from Shrang's argument). Shrang also gave some solid points about how it was kind of your choice to be helpless to Mega-Gengar by not recognizing it as a threat that has amplified the problem. As already noted, there are still options like shed shell (blissey), dark pulse grassceus, etc. that allow you to work around or address the problem. One argument on the discussion thread highlighted it as "addressing the threat instead of hoping it was banned". On that note, my final thought towards Mega-Gengar is that instead of banning it, perhaps it should be treated as a notable threat (just like how every team has an answer for Kyogre, e-killer, geoxern, etc.) and that the metagame should adapt instead of resisting change.