This is really a great overview of why I like this setup, I couldn't have put it more clearly.
An example of something I've been trying lately and didn't work well under OSX: ruby bindings for FUSE filesystems. I wanted to write a quick FUSE fs using ruby, so I installed gems and other dependencies. But since MacFUSE has been deprecated for a while, it's not easy to get everything working.
It could probably work under OSX, but given the current state of FUSE on Lion, I gave up (and did not try it under Linux yet).
While trying to setup my FUSE environment, I installed every FUSE version, many gems with a couple of ruby versions with rvm, rbenv, ruby-build, etc. When playing with new tools and environments, I always end up with lots of useless or broken stuff.
This setup allows me to keep my base system clean and simple.
I do not use it daily, but it's nice to have it ready to launch when needed.