It would be somewhat cool if ruby-build knew how to look for an rbenv folder to install rubies into, or if rbenv knew how to drive ruby-build to put rubies in the right directory, but otherwise it's pretty nice.
It is fairly small, but I think part of the idea is to decouple rbenv from how or with what tools you build Rubies. (It might make sense, though, for someone to create and maintain a fork that bundles ruby-build and rbenv.)
