Buentello is definitely the very top tier of "B" level fighters. He's great, very technical, I pick him over Struve and even Duffee without taking too much damage.
I think perhaps he's just not genetically gifted enough to reaaaally be a top tier fighter. As to why he was cut I have no idea, he destroyed the guy he faced after Arlovski and he's always exciting to watch. He had some exciting fights with SF and Affliction for sure, and I wish SF still had him in their roster.
Buentello headlined the first SF card he was in, so maybe that was the bargaining chip that they used to get him from the UFC.