I'm quite surprised by some of the snobbery on this thread, I can really see why it'd be useful, and I don't see why it implies that any bassist that does use one is a poorer bassist or has less theory knowledge than one that doesn't. I agree with the notion that it's basically no different to a drop-tuner, tuning to a different key or even going 5 string, and I have nothing against any of those things either.
For example, here's Martin Mendez of Opeth, a fantastic bassist, making use of a capo: