I can see the appeal of something like RVM. But despite using Ruby a lot, it's mostly stuff for myself so I just use whatever it's the latest. The reason why I thought about RVM was that I wanted to try MacRuby (and rare cases when I need to move back to the system's ruby) and was suggested to use RVM.
RVM is great to test against different versions (and implementations), especially if you do a lot of team-work: For instance, if you work in a team, or in production environments, the newest version of Ruby may be ahead of the production version, and then it's a must being able to switch from one version, or implementation, to another--painlessly.
A personal use case I could come up with (there are a lot of them): You make an app., and it runs with Ruby 1.9.2. You deploy this app., and then Ruby 1.9.3 is released, and your app. is no longer compatible. You want to stay on the cutting-edge still, so you want 1.9.3, but your project is not compatible with 1.9.3, and still needs some work to be, since it's grown rather huge. Here RVM would be handy!
When I was just toying around with Ruby, I didn't use RVM either. It's not until I've got work within the Ruby field, I've had the need for it. :)
I haven't noticed any clashes even in this case. By default your rvm is installed for just one user. So if you're all that worried I'd create a different user to try out rvm, and just install rvm on that user. Then if it fucks up you can nuke that user.
If you want to play with MacRuby and keep a more standard Ruby that you already have installed, rvm may be a good choice.