[Disclaimer: I'm not a Ruby developer, but have developed in plenty of languages, as well as being a sysadmin for a fair while]

IME, separate package-management for each and every language is more painful than the problem he is describing.

Rubygems, CPAN, whatever other languages use.

As soon as you install things outside of the distro package-manager, it has no idea what is going on with those packages and so continues to think that they aren't installed. If there was a way to get the package-managers for language libraries and the distro to play together nicely and work for dependencies etc then this would be OK, but as it is it's a bit of a sysadmin nightmare.

Edit: I thought that installing distro-supplied packages into /usr/local was against the FHS?