mod_python can use several interpreters to handle the multi-user situation, because you can name the main sub-process and bind it to a virtual host.
For the level of permissions: this is how php works too. and there is billions of php users here.(yes you can use suexec + php as cgi but you can do the same w/ python). Anyway, if you want this kind of situations (I want for part of my projects) you can use mod_scgi w/ quixote. So you can reuse, and separate.
But mod_python require apache 2, and yes this is pain to install. For example, there is no php module for apache2 in debian (there is a one for apache2-prefork but...)
I haven't put quixote + mod_python in production right now. but develop w/ quixote and medusa is really easy, and I think that: with the bunch of way to deploy quixote (medusa + apache proxy, mod_scgi, pure cgi, cgi + FastCGI, twisted, mod_python) i gonna find the best way to handle the requests.
What's wrong with apache2 prefork vs the old apache? I mean, the concurrency model is THE SAME and it's ACTIVELY DEVELOPED. Just because the PHP guys have a stick up their collective ass about it is no reason to shun it.