Theres been a lot of dont ban MSab, so this is just a post playing devil's advocate.
MSab should be banned from monotype since certain types realistically cant beat it, while every type certainly has pokemon to beat it in a 1v1 MSab is a ghost type and cant be trapped, so they are going to switch out.
Electric:
Toxic MAmphy - Umbreon has heal bell, which contrary to popular belief, actually does remove toxic status for MSab. Umbreon and MSab also get reliable recover (recover and wish) while MAmphy does not, allowing them to easily outlast MAmpharos, and if MSab even starts clicking an attacking move when MAmphy comes in then it wont last long
0 SpA Mega Sableye Dark Pulse vs. 252 HP / 4 SpD Mega Ampharos: 69-82 (17.9 - 21.3%) -- possible 5HKO
0 SpA Mega Sableye Dark Pulse vs. 252 HP / 4 SpD Mega Ampharos: 69-82 (17.9 - 21.3%) -- 9.7% chance to 4HKO after sandstorm damage
0 SpA Mega Sableye Snarl vs. 252 HP / 4 SpD Mega Ampharos: 49-58 (12.7 - 15.1%) -- possible 7HKO
0 SpA Mega Sableye Snarl vs. 252 HP / 4 SpD Mega Ampharos: 49-58 (12.7 - 15.1%) -- possible 5HKO after sandstorm damage
As you can see even bulky MAmphy sets with max hp can only switch in around 7 times into an attack, combine that with any sandstorm damage (its a special attacker, why wouldn't they switch into ttar? If you start predicting and clicking focus miss, they just keep in MSab and hit you again, making it die even faster!)
Band Guts Luxray - Firstly MSab has no business spreading burn around, what does it affect? Its a type full of special attackers, MSab would be better of a nice boost to its SpDef in using calm mind, which would let it take on the entire rest of the team. You are then left with
252+ Atk Choice Band Luxray Wild Charge vs. 252 HP / 252+ Def Mega Sableye: 142-168 (46.7 - 55.2%) -- 69.1% chance to 2HKO
Which MSab then recovers away, leaving you with a check that doesn't work.
Utility MSab also beats Luxray
252+ Atk Luxray Wild Charge vs. 252 HP / 169+ Def Mega Sableye: 102-120 (33.5 - 39.4%) -- guaranteed 3HKO
SubCMRaikou - Is a lord an beats everything, whats the surprise? #notbiased But really, it loses to snarl. +1 snarl will 5hko, and you cant just start tbolting it since then you lose
+1 0 SpA Mega Sableye Snarl vs. 4 HP / 0 SpD Raikou: 78-93 (24.2 - 28.8%) -- possible 5HKO after Leftovers recovery
252 SpA Raikou Thunderbolt vs. +1 252 HP / 0 SpD Mega Sableye: 81-96 (26.6 - 31.5%) -- guaranteed 4HKO
-1 252 SpA Raikou Thunderbolt vs. +1 252 HP / 0 SpD Mega Sableye: 54-64 (17.7 - 21%) -- possible 5HKO
Ice:
Specs Walrein - Can actually win, assuming the dark user doesnt feel like preserving something that can almost 6-0 ice by itself
252+ SpA Choice Specs Walrein Frost Breath vs. 252 HP / 60 SpD Mega Sableye on a critical hit: 165-195 (54.2 - 64.1%) -- guaranteed 2HKO
252+ SpA Walrein Frost Breath vs. 252 HP / 60 SpD Mega Sableye on a critical hit: 109-130 (35.8 - 42.7%) -- guaranteed 3HKO
(If they koff then switch out and come in and recover on something else, then rip)
Specs Lapras - Loses to MSab, it can koff then recover away the damage or even switch to a teammate.
252+ SpA Choice Specs Lapras Frost Breath vs. 252 HP / 60 SpD Mega Sableye on a critical hit: 153-181 (50.3 - 59.5%) -- guaranteed 2HKO
252+ SpA Lapras Frost Breath vs. 252 HP / 60 SpD Mega Sableye on a critical hit: 102-121 (33.5 - 39.8%) -- guaranteed 3HKO
Max rolls = 99% damage!!! (2 hp from a ko), your going to need a tiny bit of prior damage but if its a 1v1 (like all your checks so desperately want) then it loses.
Steel:
Bisharp - Loses to MSab
+2 252+ Atk burned Bisharp Iron Head vs. 252 HP / 169+ Def Mega Sableye: 91-108 (29.9 - 35.5%) -- 25.6% chance to 3HKO
252+ Atk burned Bisharp Iron Head vs. 252 HP / 169+ Def Mega Sableye: 46-54 (15.1 - 17.7%) -- possible 6HKO
Magnezone - Loses to MSab
252+ SpA Choice Specs Magnezone Thunderbolt vs. +1 252 HP / 0 SpD Mega Sableye: 144-169 (47.3 - 55.5%) -- 76.6% chance to 2HKO
Heatran - Needs hax to win, even with an active flash fire
252 SpA Flash Fire Heatran Lava Plume vs. +1 252 HP / 0 SpD Mega Sableye: 117-138 (38.4 - 45.3%) -- guaranteed 3HKO
252+ SpA Flash Fire Heatran Lava Plume vs. +1 252 HP / 0 SpD Mega Sableye: 127-151 (41.7 - 49.6%) -- guaranteed 3HKO
These are just 3 types that can have trouble with MSab, and even for these 3 MSab can beat its "checks" 1v1, or you can sack something and bring your check in but then its either too strong (rip special attackers) or its teammates easily take care of your checks
MSab by itself also steamrolls Ghost teams, as nothing can beat MSab except another MSab on that type. This is already
3 types (not including ice) that cannot beat MSab due to MSab 6-0ing by itself or because MSab with a tiny bit of team support ends up winning. This doesn't even take into account other types that cannot beat MSab + friends, and while team support alone can never ban a pokemon it really can break a pokemon when everything you have is easily beaten by the teamsupport.
Apparently MDoom does beat MSab, ( thx
Joshz ) but standard ttar can counter it
56 Atk Tyranitar Stone Edge vs. 0 HP / 0 Def Mega Houndoom: 318-374 (109.2 - 128.5%) -- guaranteed OHKO
+2 252 SpA Mega Houndoom Fire Blast vs. 252 HP / 200+ SpD Tyranitar in Sand: 95-112 (23.5 - 27.7%) -- 83.5% chance to 4HKO
(Not that MDoom should stay in to Ttar anyway, so Sabless dark can beat MSab dark)
When taking into account teamsupport that are suddenly more types that cannot beat MSab, as pokemon that are easily worn down by hazards, sand storm damage, and just attacks in general when switching into MSab eventually die, and MSab + friends keep healthy with wish, recover, roost and lefties.
What about ghost? It has to be a casualty. The same principle that applied to Skymin and KyuW apply here. When they got banned the council said that they wouldn't be unbanning any more ubers. Why? Because they are broken in the monotype metagame. The same applies to MSab, just because Ghost becomes bad doesn't mean you shouldn't ban a broken pokemon, and even though offensively it will never reach the same potential the same affect still applies.
Just to end this I would like to point out how MSab is broken and uncompetitive from ous tiering policy.
http://www.smogon.com/forums/threads/ous-tiering-policy-framework-read-and-understand-this.3552154/
"II.) Uncompetitive - elements that reduce the effect of player choice / interaction on the end result to an extreme degree, such that "more skillful play" is almost always rendered irrelevant
A.) This can be match up related; think the determination that BP took the battling skill aspect out of the player's hands and made it overwhelmingly a team match up issue, where even with the best moves made each time by a standard team often were not enough."
In monotype the matchups are pretty skewed, much more than in ou. However certain matchup advantages get to be too much. MSab auto wins against electric if they use MAmphy as their check with toxic and the dark user carries heal bell. If MSab carries the right set and teammates it can easily get past teams that rely on 1 check to beat it. This fits with the BP team example, and while that may be ok for a few types (dragon v electric or fire) it is unacceptable for a ton of types (dark v electric, ice, steel, dark, ghost) to be auto losing based on who their check is and whether the dark user prepared for it. In this way MSab reaches the "uncompetitive" status.
"III.) Broken - elements that are too good relative to the rest of the metagame such that "more skillful play" is almost always rendered irrelevant
B.) Examples are mostly Pokemon and include strong Ubers like Kyogre, Groudon, and Arceus. These aren't necessarily completely uncompetitive because they don't take the determining factor out of the player's hands; both can use these Pokemon and both probably have a fair chance to win. They are broken because they almost dictate / require usage, and a standard team facing a standard team with one of them would be at a drastic disadvantage.
These examples limit team building skill.
C.) Examples also include ones whose only counters or checks are extraordinarily gimmicky Pokemon that would put the team at a large disadvantage elsewhere.
These examples also limit team building skill.
D.) Uncompetitive and Broken defined like this tend to be mutually exclusive in practice, but aren't necessarily entirely so.
1.) BP was deemed uncompetitive because of how drastically it removed battling skill's effects and brought the battle down to match up, but it could also be deemed broken because of the unique ways in which you had to deal with it.
2.)
While this isn't always the case, an uncompetitive thing probably isn't broken, but a broken thing is more likely to be uncompetitive simply due to the unique counter / check component. For example, Mega Kangaskhan was deemed broken because it was simply too good relative to the rest of the metagame and caused the tier to centralize around it, but it could also be labeled as uncompetitive because of the severe team match up restriction it caused by punishing players if they did not pack one of the few gimmicky and obscure counters or checks for it."
MSab also fits under the definition for broken. While it may not be a uber capable of 6-0ing a majority of teams, it is broken in the way it requires something out of every type. While this is applicable in a small way to a lot of pokemon, such as how a water user must always prepare for Mega Venusaur, MSab pushes this to the extremes. A lot of types must include a check to MSab, just like water needs something to get past MVenu, but MVenu and water is just one type. MSab requires a majority of types to prepare for it, and more often than not creating specific sets just for MSab. While a little bit of change and adapting is good for a meta, a pokemon that forces every user of a type to use x as a check to not get 6-0d is broken. You cannot skill your way past MSab, as the only way to get past it is to use your check and click a move, no amount of double switching will force it out, no amount of surprise moves on physical or special attackers will kill it, you must use a certain pokemon or die.
Another thing to note is that banning MSab can only make the meta healthier. It greatly lessens the teambuilding pressure put onto teams, which would lead to a more fun and more diverse meta. Ghost and Dark still have regular sableye, which is also better at checking physical attackers but unlike the mega cannot wall an almost infinite amount of special attackers with cm + snarl.
This is why MSab should be banned, its combination of teammates, moves, abilities, and bulk make it broken and uncompetitive, and its difficulty to even check is shown in the way users prepare for it.