What's wrong with only installing Python/Perl/Ruby if the native one doesn't satisfy the dependencies, as opposed to (currently) installing it every time?
The problem is that the versions of the non-macports Ruby/Python/etc aren't managed by the ports system. So you're in for weird dependency mismatches if you upgrade your system version of Python.
Self-contained keeps it simpler. Just use a script to set up your $PATH as needed.
The people who tried to get the Gentoo Portage system running on OS X a few years ago tried that idea. It ended in tears.