Is Gengar worth powering up?

Since Alakazam solos don't exist anymore you'll only be using him against Mewtwo, and only against the Psycho Cut variants, and not with shadow ball/ psychic either.

I wouldn't be too bothered about the legacy move- in some cases Hex actually runs ahead if you do decide to use it against confusion mewtwo because of the 4 confusions and die pattern. Hex can charge the shadow balls fast enough while Shadow Claw may not (and so it may come up one shadow ball short and lose the DPS race).

That's the only case I can think of for now. I maxed a 98% but it is a legacy and I caught it from level 30 so less investment needed than an egg hatch, to max yours would be a considerable investment.

You'll probably end up using him when Poison becomes important in the game... okay, so he's a Trophy.

But he's a 100% Trophy! And you MIGHT end up with a 100% Pokémon that can Mega-Evolve if Mega-Evolving comes into the game.

I was disappointed I got sucker punch/focus blast on a couple really nice high CP/high IV evolutions - BUT, I'm finding that odd combo is terrific against Snorlax and Blissey!
So even though the movesets seem bad, he's still really useful.

Yeah shadow claw is gone, but Gengar's high attack stat almost makes up for it!