The point is that building a new Python just to generate virtualenvs from is not disruptive to anything else. Zed says they had to move away from Python because the system would get messed up if you replaced the distro-supplied Python. Building a new Python and merely using it from the build directory to generate virtualenvs doesn't disrupt anything in the system. We have done this on several CentOS systems and had no issue.
You just build it and don't install it, run it from the build directory or some other isolated space that won't effect the system. You _can_ create venvs from there. After you have done so, just carry your venv around with you and there's no disruption from anything. Our venvs are all kept in project-top-level/venv.
But if you rebuild python from scratch, you loose every single software packaged by your distribution. If you depend on modules with C extensions, it quickly becomes intractable. Building python is easy - building a recent pyqt on RHEL 5, not so much.
Also, virtualenv is hard to sell for people who do not know so much about python.
You just build it and don't install it, run it from the build directory or some other isolated space that won't effect the system. You _can_ create venvs from there. After you have done so, just carry your venv around with you and there's no disruption from anything. Our venvs are all kept in project-top-level/venv.