I strongly agree with what Byron said about the iterative learning process. Too often I find myself rewriting core functionality as I gain more insight into coding and as the engine takes shape and it's possible to see how the various pieces fit together. People who are writing their third or fourth game engine probably don't suffer from so many rewrites, but at the beginning it's often necessary to be able to do so - which is why automatic refactoring is really great. I'm not sure if there's support for this feature in the Flash/AS IDE though.