There's bike vandalism to watch out for, too. It's not very common, but few locks protect against it. Locks handle a much more common occurrence, that people want to steal a bike for material gain, not only for the detriment of the bike's owner.
DC has replaced some bike racks with bike lockers that totally enclose the bike. The downside is the lockers take up a lot of space so there are only a few of them at each metro station.