The problem I see with having Hulk as the main character, is the fact that he, by nature, has to try and be a neutral, laid back kind of guy, to be a human. And having main protagonists in movies that, by design, strive for The Dude in their mental state, doesn't make for a really action intense character. We can't see them get angry at an injustice (without Hulking out and causing destruction), we can't see them...well, basically emotional in any way. So, you end up having The Dude for the talky scenes...and then switch to Walter when action comes around. Neither character really holds up on their own, as they are, by their very nature, 2 polar opposites of a normal person.