Poison is not at all good as an attacking type, especially in OU, with all the Steel and Ground types running amok. Its only super effective against Grass, which isn't all that common in this Dragon-infested meta(the only widely used Grass type, Nattorei, is immune to poison). On the other hand, Ghost is super effective on other Ghosts as well as Psychics while only being resisted by Dark/Steel/Normal. Of these, the most used Dark type(Tranitar) is beaten handily by Focus Blast, all prevalent Steels either take neutral from Shadow Ball or are handled by Focus Blast, and Normal types are not common at all anyway.
EDIT: >:(