In mature production environments, sometimes you do need more than one Ruby environment on a host. I've seen this case where multiple programs must be deployed on a host, one of which was built a long time ago (i.e. legacy). (This applies not only to Ruby, BTW.)
Maybe rvm/rbenv is not the best solution, but some solution is needed. You can either invent one yourself, or adapt something that's already been built. We're using rvm, and while I'm not a huge fan of it and would rather switch to rbenv, I've managed to tame it so that it's not such a huge beast, using various wrappers so that non-interactive Ruby programs work start up in the correct environment.
My money's solidly on ruby-install for this, but only to build packages - not to run in production. You just end up with rubies in /opt/rubies/ruby-X.X.X-pXXX/bin. Picking one is as simple as plonking that on the front of $PATH.
If the divide is ruby 1.8 and 1.9, Debian packages both, and they can be installed simultaneously.
If it's more arcane than that, it sounds like kvm or lxc is the answer.
If you reallyreally need to run two pieces of software with differing ruby interpreters on the same machine, then your real problem is technical debt. One fix would be to update the ancient software so that it can use modern Ruby. Another fix would be to re-architect the solution so that the greater system could operate across multiple OS instances. The least attractive option would be to use packaged Ruby for the modern stuff, and a hand-compiled Ruby for the legacy stuff.
If you really really need to run two pieces of software with differing ruby interpreters on the same machine, then your real problem is technical debt.
Why?
Why can't you have a stable app using Ruby 1.9 that hasn't need any updates (other than security) in three years running on the same machine that also runs another newer application which uses Ruby 2.0 and Rails 4?
Maybe "wasting asset" is a better metaphor. At some point, there'll come an OS release on which that Ruby won't even build (and this can happen sooner than you'd think - 1.8.7 needs to be patched to build on Wheezy, for instance; by the time Jessie goes stable 1.9.3 will probably have been deprecated and unsupported too). At that point, you're forced into porting to a newer Ruby just so you can keep up with security patches to the underlying OS. It might not be "technical debt" per se, but it's definitely a maintenance cost.
kvm or lxc are interesting, but that seems like attacking a fly with a sledgehammer, and it's hardly cost-free in terms of monitoring, maintenance and resource economization. Why bother with virtualization when setting path-related environment variables is sufficient?
Maybe rvm/rbenv is not the best solution, but some solution is needed. You can either invent one yourself, or adapt something that's already been built. We're using rvm, and while I'm not a huge fan of it and would rather switch to rbenv, I've managed to tame it so that it's not such a huge beast, using various wrappers so that non-interactive Ruby programs work start up in the correct environment.