The main use case is for specifying per-application Ruby version dependencies. For example, at 37signals, most of our apps run on REE, but our new apps run on 1.9.x, and we're gradually moving everything to 1.9. When you have multiple people working on multiple apps every day, it's essential that this dependency information is checked into version control. Even more so when certain branches of an app may depend on different versions.

Both rvm and rbenv allow you to specify per-application dependencies (rvm with .rvmrc files, rbenv with .rbenv-version files). The difference is that rbenv does it in a much simpler, less invasive way.