Rbenv and RVM provide a lot more than just four major revisions of Ruby. They provide explicit, exact version selection down to the exact patchlevel of the release, alternative interpreters like JRuby, Maglev, Rubinius, and Ruby EE. Plus, they are user-installable, and have tools for automatically managing selecting the correct Ruby for a project without having to issue special commands.
Certainly I'd prefer if popular Linux distributions made more versions of tools like Ruby available as packages, but just having Ruby 2.0 and 2.1 installed at the same time is not the only point.
Honestly, the rbenv model is looking pretty good these days given that there's also pyenv, nodenv, and plenv, all direct forks of rbenv. For the servers I take care of, I set up system-wide rbenv and pyenv installs as a non-root user to give our applications the flexibility of these tools without having to screw around with root access where it's unnecessary. It works great, and I'm planning to move Perl to the same model.
If you would be willing to chat about such issues on the IRC channel #perl6 on freenode[1] (usually busiest from around 8am to 8pm EST), especially with FROGGS and/or lizmat, you could help the Perl 6 team as they specify and implement package management that tackles these issues (multiple compiler versions, multiple module versions, alternative implementations by different authors, CPAN/PAUSE, github, local repos, and so on).
Certainly I'd prefer if popular Linux distributions made more versions of tools like Ruby available as packages, but just having Ruby 2.0 and 2.1 installed at the same time is not the only point.
Honestly, the rbenv model is looking pretty good these days given that there's also pyenv, nodenv, and plenv, all direct forks of rbenv. For the servers I take care of, I set up system-wide rbenv and pyenv installs as a non-root user to give our applications the flexibility of these tools without having to screw around with root access where it's unnecessary. It works great, and I'm planning to move Perl to the same model.