I agree, RVM is a fantastic tool and a god send. Tremendous job by the creator. I just always felt that it was 'odd' to have to install N version of Ruby just to work on a particular project.
RVM is a very impressive collection of monstrous hacks jammed into a kitchen-sink tool to achieve a set of ends that can be better met in other ways. It's worth learning enough about how the Ruby environment fits together to figure out how. It's great to get learners off the ground (assuming it works first time), but if you're doing ruby seriously then I reckon part of your education should be how to get off it as soon as possible.