I have used crobat but I think poison types are just fillers
I think you might be right. There is no pure poison type; they are always mixed with another. But some combinations can have advantages. Take Drapion for example. With his Poison and Ground combination he only has one weakness: Ground! But he can learn moves to counter that, like Ice Fang. So I guess it would depend on what Poison type it is. Like for me, Beedrill is nothing.
Hope this helps =)