The ability to install multiple versions of the same gem are pretty much key to the migration path of any ruby app. Just because it doesn't fit in the package manager world of some (read: all) distributions is not the fault of Ruby.

I can't count the number of times that we rolled out (while I was at the AJC) a new version of one of our apps on a different version of rails while keeping the other apps collocated on the same box running the previous version.