They probably didn't know about these microbe's effectiveness. That, and there was so much publicity about the problems that they had to do ~something~ --- doing nothing would have been taken even worse by the public. Seriously, imagine BP saying, "uh ... yeah, there's not a problem, billions of tiny microbes are going to take care of our accident." People would have mocked them plenty, and their PR guys wouldn't want that.

Well, it doesn't seem to have hurt this process (note that a large fraction of Corexit is light petroleum distillates, i.e. more chow for these bugs) and at least one source speculated it helped by breaking the oil into small droplets. The more you increase the surface area the more access bacteria can get to it and to the extent its application was effect it likely made a big difference.