I always thought that the Gym Trainers were part of the gym, not trainers about to fight the gym. For instance, there is a trainer in the first gym that says "you're light years away from facing brock" and in the second gym yet another that tells you "misty can wait". Seems like they're teamed up with the leader.I always thought they were aiming for a ninja based gym, psychic and poison would be a part of ninja based moves. I think i remember Koga using Muks and Weezings, and some poison in the actual trainer teams. But you have to remember your not the only trainer collecting badges, other trainers could just be challenging the gym, hence why they have psychic, they got the wiser and used poison's weakness.
If they were going for a sneaky or ninja themed gym, I wonder why Scyther didn't appear.
On subject though, it's not like they were strapped for choice when it came to poison Pokemon. Odd how no Gastly/Haunters were present in a gym like that.