Comment 3 for bug 488681

To be honest bzr-svn is also has similar problem: it blocks running selftest if there is no subvertpy library installed:

C:\work\Bazaar\bzr-2a\bzr.dev>py26 bzr selftest --list-only > NUL
bzr: ERROR: No module named subvertpy
You may need to install this Python library separately.
bzr: warning: some compiled extensions could not be loaded; see <https://answers.launchpad.net/bzr/+faq/703>

Why Explorer is any worse then?