I vote to
not ban gengarite/mega gengar from the ubers tier.
Full disclosure, I am primarily an ubers player who dabbles in OU and other tiers, so please don't hate on me too much if I say something stupid about OU (see 6 and 8) in this paragraph/essay/dissertation/novel. ;)
A few reasons why not to ban:
1. Gengar/Mega gengar is not a switch in to anything. And you'd like it to be so you could trap things like gothitelle does.
I don't think I will find many arguments that before it mega evolves gengar is one frail pokemon. It is frail in OU never mind ubers, so I don't think I'll get much argument there. Of course it does get a slight bulk increase to defenses of 60/80/95 once it mega evolves but that is still not all that great in a tier where 120 is the minimum your attack or special attack stat that any respectable ubers pokemon should be hitting, and usually most self-respecting special attackers in ubers have at least a base 131 special attack stat. Take for example a weaker special attacker. Weaker than Kyurem White, Kyogre, xerneas (scarf psychic makes gengar sad and if you haven't used scarfed xerneas you should it wrecks), deoxys-a, mewtwo, palkia reshiram. Take uninvested unboosted arceus. Arceus, while an amazing poke has only a base 120 sp attack stat, is less or much less than the more conventional special attackers I listed above. So let's do a calc:
0 SpA Zap Plate Arceus-Electric Judgment vs. 28 HP / 80 SpD Mega Gengar: 145-172 (54.1 - 64.1%) -- guaranteed 2HKO. Univested this poke two hit KOs mega gengar, even if it has already mega evolved and has some bulk investment and is hitting mega gengar on its bulkier special defensive side. So if you can't switch in mega gengar you basically need a free switch into it and therefore you usually have to let a pokemon faint (they don't die, they just faint), so if you have to destiny bond with mega gengar to take something out that is two pokes you've lost just to get mega gengar to work which is not a good trade for you. Remember the above calc is if mega gengar has already mega evolved, which brings me to my next reason why not to ban.
2. Gengar with gengarite has to mega evolve to gain shadow tag.
This in some ways makes it easier to play around than gothitelle. Gothitelle can switch in on support arceus going for recover and you're trapped. Gengar not yet mega evolved tries the same thing and then you can still switch that first turn, into say banded ho-oh (good luck switching into that monster), pursuit trap aegislash/scizor, or a scarfer. This is a big disadvantage for mega gengar. Another reason not to ban.
3. Who cares if mega gengar is over centralizing?
I don't buy the argument that mega gengar over-centralizes the tier or limits team building, because even if it did, which is debatable, that would be okay. This is ubers. This tier/ban list is already very over centralized. Take for instance Kyogre. It is very difficult, to not run a kyogre, counter (rest talk dragon tail sp def palkia, arceus grass or arceus water, the latter I have less experience with), kyogre switch in or kyogre check on every team you make. That is just the nature of ubers. Kyogre limits team building. E killer arceus limits team building. Xerneas certainly limits team building. If mega gengar limits team building, so what? Powerful yet competitive threats have been apart of ubers for as long as I've played the tier.
4. Gengar does not make stall unplayable.
Stall is incredibly powerful. I've watched many a high level tournaments and seen stall due so well and top players really use it successfully and while you now have a threat to stall in mega gengar. Gengar by no means beats stall all by itself or makes stall unplayble. Take for example this battle where I'm using one of Donkey's stall teams (I usually don't run stall but I wanted to try it because it looked fun) and I'm battling a very respectable user who had a mega gengar. And by very respectable user I mean rarecandydrugger who as I type this is #2 on the ubers ladder, and in the very recent past was #1. Now I should lose right? I should just forfeit because I heard mega gengar just destroys stall. And this is mega gengar in the hands of a good player and I don't even have a pursuit aegislash or scizor, oh no! What am I going to do?
http://replay.pokemonshowdown.com/uberssuspecttest-156608043
Well of course I won and it just goes to show that stall is not completely dead because mega gengar is allowed.
5. Gengar is not swagger/moody/evasion pass(?)
When I think of bans in ubers, I think of abilities/moves that are totally luck based by nature. Swagger: 45% chance your opponent will not be able to do something the turn you use it and 50% until your opponents pokemon breaks out of confusion. OHKO: Fissure 30% chance the opponent faints no ifs ands or buts. Moody: made bidoof viable in ubers. These are the moves and abilities that turn the game into a coin flip/dice roll. They are no fun (in my opinion) and take no skill, they're just luck. That's why they are banned. Mega gengar you have to play around, sure, but that takes skill. I admit that sometimes taunt destiny bond can be a similar situation, but I don't think you can make an argument that mega gengar is any where in the league of swagger/OHKO/moody etc. Unlike these other moves and abilities, gengar needs so much more help, such as a free switch, a turn to mega evolve, not getting smacked upside the head by a choice scarfer, pursuit user or shadow sneak, to work. Really, I think this would be the only real reason to ban a pokemon from ubers is if it is luck based or "uncompetitive." Gengar, in my opinion, is not luck based like some of these more bannable/moves and abilities. It just takes skill to play around.
6. Mega Gengar can't have an item besides its gengarite.
This inhibits gengar in a lot of ways. First it makes it less appealing for gengar to run substitute because unless you run substitute (sub)/pain split you're slowly killing yourself and if you do run these two moves then you are really limited in terms of coverage. And don't tell me gengar wouldn't like to run sub. Sub is practically a staple of gengar in OU because it mitigates to some extent its pathetic bulk. Second Gengar is somewhat more predictable because I always know what item it is going to use. I know it is going to have gengarite. I know my choice scarfer out speeds it. I don't have the same luxury with most other mons. Is that kyogre scarfed or specs? Is that ho-oh banded or sub? What kind of arceus is that? I don't know. But when you see gengar you at least know how fast it can be and how hard it can hit. Third gengar can't have a life orb and gengarite at the same time. So what you say? So, I hear a lot about how powerful gengar is but consider the following (said one Bill Nye the Science Guy): "252 SpA Life Orb regular old run of the mill Gengar Sludge Wave vs. 252 HP / 0 SpD Arceus: 173-204 (38.9 - 45.9%) -- guaranteed 3HKO" and "252 SpA Mega Gengar Sludge Wave vs. 252 HP / 0 SpD Arceus: 162-192 (36.4 - 43.2%) -- guaranteed 3HKO." Regular old gengar with a life orb, hits harder than mega gengar. Gengar, while it gains speed when it mega evolves basically loses potential power. This is why I don't every buy the argument that gengar is too powerful, when I hear "as much special attack than kyurem white" etc. Fourth, mega gengar takes up the mega slot for your team. No mega lucario, kangaskhan, or blaziken for you. Also, soon this will mean no mega kyogre or groudon. So this is a non-trivial opportunity cost for running mega gengar.
7. Knock off
As I'm sure you already know knock off is really popular this gen and it absolutely destroys mega gengar due to its physical frailty and weakness to it. Fortunately for mega gengar the only two real spammers of knock off in ubers are lando-t and blacken. However gengar would really like to switch in to landorus-t otherwise on a predicted eq, stealth rock or toxic but it just can't as freely because of knock off.
8. In
my experience on the ladder, people have spoken with their feet (or more precisely their precious team spots) and said that mega gengar is not that great
If mega gengar was so good than wouldn't everyone be running it? Now I know that usage is not how we are supposed to be evaluating a pokemon for a potential ban, which is why this reason is 8th on my list and not 1 or 2. But I remember when life orb tornadus t was spamming hurricane and it was on every single team before its ban in 5th gen ou and how mega kangaskhan and mega lucario was on like every team in 6th gen when they were allowed. Their usage clearly signified that something was broken about those mons and I just have not seen the same thing happen with mega gengar. I'm saying it hasn't flooded the ladder like I would think something that is so broken it needs to be banned should. And if it is going to be banned from ubers it better be super worthy.
9. This is ubers. We don't ban pokemon. (Just abilities and moves)
This is more of an opinion than an argument but I think that it lies at the heart of what I think the ubers tier is. An over-centralized ban list of super powerful pokemon. And, in my opinion, mega gengar fits right in. It's not uncompetitive just powerful.