Gengarite: Do Not Ban
Assuming "one (1) paragraph of reasoning explaining your choice" does in fact mean one paragraph (i.e. several sentences), I'll try to sum up the main reasons I feel it shouldn't be banned in several sentences.
Mega Gengar does not automatically remove a Pokemon of the user's choice, as it supposedly does in theory. Defensive Pokemon that are weak to it (e.g. RestTalk Xerneas, Blissey) can viably run Shed Shell to escape from it ("overcentralization" in terms of preparing for a top threat is of zero importance in a tier where Kyogre, or Xerneas in XY, is on every other team), while other defensive Pokemon like Ferrothorn either beat it if it lacks the appropriate coverage or force it into a Taunt/Destiny Bond 50/50 (which, in case it's unclear, goes both ways). There are also Pokemon that can naturally escape from it by virtue of Ghost typing, U-turn, etc. Even if Mega Gengar removes its intended target, it often does so at the cost of fainting. I get that this makes it easier for a teammate to sweep (which is hardly any different from how a wallbreaker functions), but using Destiny Bond still comes at a cost to the Mega Gengar user. Gengar also has to take a turn to Mega Evolve and gain Shadow Tag, which is pretty self-explanatory and something that can't be overlooked. It's also a relatively average Pokemon against offensive teams (mainly because of its initial lack of speed, though, which isn't even related to why it's being suspect tested).
In other words, not only can the "removal of choice" be circumvented, but it doesn't guarantee that Mega Gengar will actually KO what it needs to, and it certanly doesn't guarantee that Mega Gengar will KO what it needs to without cost.
Assuming "one (1) paragraph of reasoning explaining your choice" does in fact mean one paragraph (i.e. several sentences), I'll try to sum up the main reasons I feel it shouldn't be banned in several sentences.
Mega Gengar does not automatically remove a Pokemon of the user's choice, as it supposedly does in theory. Defensive Pokemon that are weak to it (e.g. RestTalk Xerneas, Blissey) can viably run Shed Shell to escape from it ("overcentralization" in terms of preparing for a top threat is of zero importance in a tier where Kyogre, or Xerneas in XY, is on every other team), while other defensive Pokemon like Ferrothorn either beat it if it lacks the appropriate coverage or force it into a Taunt/Destiny Bond 50/50 (which, in case it's unclear, goes both ways). There are also Pokemon that can naturally escape from it by virtue of Ghost typing, U-turn, etc. Even if Mega Gengar removes its intended target, it often does so at the cost of fainting. I get that this makes it easier for a teammate to sweep (which is hardly any different from how a wallbreaker functions), but using Destiny Bond still comes at a cost to the Mega Gengar user. Gengar also has to take a turn to Mega Evolve and gain Shadow Tag, which is pretty self-explanatory and something that can't be overlooked. It's also a relatively average Pokemon against offensive teams (mainly because of its initial lack of speed, though, which isn't even related to why it's being suspect tested).
In other words, not only can the "removal of choice" be circumvented, but it doesn't guarantee that Mega Gengar will actually KO what it needs to, and it certanly doesn't guarantee that Mega Gengar will KO what it needs to without cost.
