I think that the experience thing largely has to do with being able to draw on your experiences in the work world to add to the class environment. I see two major benefits:
1. B-school is all about DOING stuff. It's not an intellectual exercise. If you've got no experience, then the coursework will simply be less relevant. I can't tell you the number of times I've sat in the classroom and had the concepts made clear by comparing them to something that I did or that I saw back on the job. Almost nothing from college ever relates.
2. B-school is also about learning from your classmates -- professors often ask if anyone has had experience in Y industry with X issue. If you don't have experience, you don't have much to bring to the table.
Obviously, these are generalizations. Your internships may give you this type of experience. And it's not like we all come to school with the perfect package - we're all missing something. But these two issues seem pretty key to me.