I use my chain far more often than either of my U-locks simply because I've had times where I couldn't find a spot on the rack or a nearby meter that would work with a U-lock. So, even a Fahgeddaboudit would have been worthless if it was the only lock I owned.

That might not be a problem for most people, though, but some places around here are so bike-heavy that you'd have to be ready to lock to a tree or a lamp post.