The problem is that rst plugin uses some feature (for example, iterator comprehension) which is unavailable on old version of Python.
So rst plugin will not work on a machine which has an old version of python in system path even though
the user have installed a new version of python in other place. For example, I am using ikiwiki with the rst plugin on Mac OS X 10.4 which ships python 2.3 but I do have python2.6 installed on /opt/local/bin/python (via macports).