I tested ahoreth's plugin a couple of times this summer and I've got to say that it's everything I would expect core to do if they introduced revisions to the theme and plugin editors. It has the added benefit of preventing breakage while editing files, just like Core guards against activating plugins that would error out.

That said, should code revisions be added to the WordPress Core? I say yes, but not yet. This is one of those features that should probably soak as a plugin for a while to gauge interest. However, It seems like a logical next step in the progression of unifying the admin experience for users.

It's important to remember that most non-developers probably have no idea what versioning is, so the idea of "cowboy coding" is beyond them. Integrating something like revisions in this space could prove to be nothing but a good thing for all involved.