Does this also apply to end-users of Ruby apps that are just an apt-get away? I don't really want to learn all that stuff (and remember to redo it on all installs) just to use some tool that happens to be written in Ruby.
I don't believe there are any rails-apps-as-packages in the official debian/ubuntu repositories, but if there were I assume they would use bundler to bundle their gems internally.
Yes there are, in our case Redmine. A pretty popular piece of software I believe. In Debian it's in main and in Ubuntu it's in universe.
Re. bundler/gems, I don't know what those are - the file "core_ext/hash/conversions.rb" I hand-patched was from a package called ruby-activesupport-2.3 which is a dependency of the Rails package.
It was redmine I was using when I had the issues actually. The real problem though was that I was trying to use a newer version of Redmine than was available in the repo, and I did still manage to satisfy the dependencies but upgrading my Ruby version broke literally everything.
I think if 100% of your eco system is from the package manager you would be fine, but if even a single component needs to come from outside I would reach straight for rvm and bundler (no prejudice against rbenv, rvm is just what I use)
Gems are ruby packages, and bundler is a way to use specific versions in an app, independent of what versions are installed globally. I think bundler would be a good fit for redmine, just because you don't really gain anything other than disk space by being able to share ruby-activesupport-2.3 between apps.