Exactly. Exactly exactly exactly exactly.Well, my opinion is very simple. Mega Gengar is not best used as a special sweeper, but most likely a revenger killer, and makes a great revenger killer in OU, teams just be prepared to face a threat like mega gengar, but I don't see the meaning of banning gengarite. Loses immunity to ground types, the stats are not very different of a life orb gengar, and the ability itself only important in revenge killing. Gets a little bulkier, but seeing the all picture, I desagree with gengarite' banning.
Revenge killing is something that is important in all Pokemon games. It is a fact of Pokemon.
However, guaranteed revenge killing is different. In all other situations, if there's a mon you have out that is being threatened by a revenge killer, you have two options. Take it like a man, allowing your own revenge killer to come in, or switch out and save your mon for another time down the road.
Now here we have a revenge killer with SHADOW TAG. Now, instead of two options, you have one. You want to save the mon for another time? MGengar goes "lolnope" and shits all over your mon. Sure, you can send in your revenge killer after. MGengar can now switch out and then do the above ALL OVER AGAIN.
I feel like a lot of posters here have no idea why Shadow Tag is such a good ability. "Oh, I can't switch, big deal." My friends, it IS a big deal.