site-packages should be used for system wide, stable packages. I try and use mostly things that are in Debian. That way I am pretty sure the package is useful, stable, easily installable(and upgradable, removable), and of a certain level of quality.
I recommend any finished package go through the getting into debian experience. It is a good peer review, and an extra set of bug testers.
Requiring root to install random-buggy-incomplete-python-package is quite bad too.