I also think rewrites happen because each of us has weaknesses, or blind spots, in our writing, and it is often helpful to bring in a fresh writer who may be able to help illuminate and solve problems in those areas. It doesn't always work that way, but I think that is the intent sometimes.

Don't forget the upside that a rewrite might actually help. Say I'm working on a comedy with a strong concept and developed characters (you know what I'm working on). You think I'm going to cry if I hear Patton Oswalt's been brought on to punch up the jokes?