Gengar is not completely vulnerable before Mega Evolving. Gengar alone is already a great Pokémon that can exert pressure, at least enough to make the opponent switch out with something that is threatened and give Gengar a free turn to Mega Evolve. We all know how good Gengar was in BW (specially SubWoW Movesets, which are also usable on Mega Gengar), and except for Perish Song Movesets it doesn't change very much except for the item before it Mega Evolves. It really isn't a "free turn".
And the point here is that unless you send in a Gengar check that can Pursuit and take a hit (that also won't work against SubDisable), your opponent can switch out, and then the battle will flow until your opponent sends in Mega Gengar. You can't switch out anymore, and anything that may be weak to Gengar will be crushed along the battle. You can't react to having your team weakened and exposed to other threats. Remember: the thing about Gengar is not its sheer power or if it can 6-0 a team, but its great ability to destroy defensive cores and almost any threat it wants in the tier.
By the way, is it me or the best points are about suspecting/banning Gengarite, while in the poll the "No" option has more votes?
A lot of people are voting "No" in order to have a suspect testing. I don't have a problem with that, this is the first ban of a Mega-Stone we could have and we need to consider all possible scenarios once Pokebank is out.