Wait, are people seriously miffed over Gengar being part Poison? The Poison sub-typing is like... more important than ever, haha. It givesGengart a resistance to Fairy (which is a big, big, big deal) and the ability to murder Fairy-types with Sludge Wave, along with a resistance to Grass and a quadruple resistance to Poison and Bug (the latter of which made it handy at taking powerful U-turns in a pinch). The only downside to having that poison subtyping is a weakness to Psychic, which... doesn''t even matter in practice. Gengar destroys most Psychic-types, and would have ordinarily lost to faster Psychic-types anyway (since they almost always carry Shadow Ball).
Ghost / Dark Gengar would be way, way, way worse.