From the comments (pleasing David Wright)

There seem to be an awful lot of arguments floating around the economic blogosphere lately that try to use “credibility” as a kind of magic trick to claim that some institution can get some desired result without having to do the yucky things it would have to do to, you know, actually get that result. I would love to see a post on this topic from our host.

That was from David Wright…and now he has his post.