At the university where I teach, the students also have to keep their bikes locked to outside racks, but I've never heard of any significant damage being done to any of them. (And I'm pretty sure I'd hear about it if it happened, since my students generally know I'm a cyclist.) But, our campus is several miles outside of town, so there aren't a lot of non-campus people who bother to come out there--though it does happen occasionally.

On the other hand, I'd be hesitant to keep a nice bike outside all the time, even if the risk of gratuitous damage is minimal. You've still got rain, thunderstorms, snow, and possibly high winds to think about.