Roy reminded me about one of the fundamental things about iterative development: keeping your bug fixes around. Otherwise you'll just make the same mistakes over and over again as you refactor. OK, so that means gathering cruft and band-aids, right? Nope ... that means knowing absolutely 100% when something is fixed. Right now the fix may be a band-aid, but later it may not. You don't really care as long as it is works. But how do you know the fix works?

Testing the API is the only way to be sure ... and since manual testing is a PITA, unit testing is the way to go. These tests become bug fix regression tests and bugs you've seen before won't creep back without you knowing about it.