> Does the PythonInterpreter option create multiple interpreters
> within a single process

Yes.

> They aren't truly isolated interpreters and nobody I've asked has yet 
> provided a use case for it.

If you ignore mod_python and mod_wsgi, then maybe, but mod_python is
*the* use case for this. Running separate process for every web app
and/or every virtual host on your server is expensive in terms of RAM
usage (and this matters if you use virtual server - 256MB or less is not
unusual). On the other hand, you need isolation for independent apps --
some modules may use globals, or you may want to be able to run both
production and testing versions of the same app (i.e. different versions
of the same Python module).