Maybe Gengar might actually have a use for sludge bomb?
Poison-type moves in general will serve a purpose besides just STAB (and a chance of status effect, albeit that's usually inferior to Toxic). Even back in generation 1, when they still had the advantage over Bug, they could still only do super effective damage against types that Flying and Fire already had covered (except Fire and Flying also had at least one more type that they'd be advantageous against), to say nothing of the fact that it hit a lot of types for not very effective damage (it was essentially Normal, except with one advantage, an added disadvantage to Ground and itself, and Ghost and Steel switching places in terms of being not very effective and being ineffective; the fact that something is immune to it at all still being a pretty big deal).
On another note, the Kebia Berry might see some more usage. More-so if a Fairy/Grass-type Pokemon is released.