Poison got nerfed pretty bad, and never really recovered.
It used to be super effective against Bug types, but from Gold/Silver on that was removed. Then they added Steel which was immune to Poison and it reduced an already lackluster attack type to just being for Toxic (although to be fair, Poison didn't have any good attacks until Sludge Bomb and other things were introduced much later).
As a Defensive type, it can be very good as long as the user can avoid Earthquake. So Weezing, Gengar, and the Poison/Flying Pokémon are all good choices there.
As for Grass? Eh... that's not good defensively or offensively, which in itself makes Poison even less useful since that's all its good against.