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.
That's a good idea... ruby-build could provide an `rbenv-install` plugin command for rbenv.
That may decrease the friction and improve the 'git checkout rbenv, add path, and get started' workflow.