Or you get a hacked up version that does not behave as you'd expect it to. E.g.: if you install gems using apt on ubuntu or debian, you'll get a modified version that tries to install rubygems from one of their own repositories. Also, it wasn't correctly symlinking gems after i'd install them either.
Apart from that though, yeah, repositories are pretty sexy indeed.
I've never even thought of installing more than one package manager. I package CPAN modules when I need them. If I needed gems, I'd probably do the same thing.
I am always weary of installing gems (actually, I use mostly Python, so I install eggs). It seems wrong to go against APT.
I can understand that, but it's easy enough to specify the apt repository as a parameter to the gems command. If you really want it as the default, it's trivial to alias that. There's something to be said for just letting your packages have the default behaviour.
And so forth.