The Limit Break was cool, but echoing everyone else's points...Renji's bankai was amazingly useless. The way it was animated it revealed that his giant whip was actually a liability against a fast opponent. It makes sense that he couldn't do anything fast enough to hit Illforte when you consider it's size, but then Illforte mentioned that his bankai was pretty fast, which didn't make sense because it looked amazingly slow, go inconsistency!

Back to the limiters and bankai, I think that bankai is more of an ability than something that requires a certain amount of 'power' to use. Thus having less spiritual energy available would probably make using it effectively more difficult, but not activating it, as Shaolon commented.