Re: Why Mark Ruffalo?
Regarding his most recent "appearance"...I had an interesting thought. Since Banner/Hulk won't have a movie of his own in Phase 2, a great way to keep him involved and relevant would be to have each post-credits scene involve him in some capacity. It would be a lot of fun to see him make secret cameos in Thor, Cap, and GOTG.
Personally, I think it would be a brilliant move by Marvel Studios.