Lady Alex
Mew is blue
Poison is quite good defensively on the pokemon who get it. It allows tentacruel to be an excellent check to scizor and lucario and gives an extremely useful fighting resist. It's especially useful on pokemon like venusaur and roserade, who resist fighting but aren't weak to ground because of their grass typing.
Gengar does benefit from it situationally. The ability to resist breloom's seed bomb is pretty good, but other than that, it doesn't matter much. Sludge bomb isn't very useful on gengar simply because of its poor coverage. Gengar already hits hard with focus blast and STAB shadowball, which have perfect coverage, and if you decide to forgo using a sub/split or sub/disable set, you're probably better off with something else still.
Gengar does benefit from it situationally. The ability to resist breloom's seed bomb is pretty good, but other than that, it doesn't matter much. Sludge bomb isn't very useful on gengar simply because of its poor coverage. Gengar already hits hard with focus blast and STAB shadowball, which have perfect coverage, and if you decide to forgo using a sub/split or sub/disable set, you're probably better off with something else still.