The API should easily allow changing the priority of fetched filetypes. Currently eggs are always preferred
This would allow using Distribute to download source Tars to set-up a development environment or to pre-process the archive. This is interesting for e.g. building executable using py2exe. In this case one needs the source archive instead of the egg.
Additionally some option "--prefer-<filetype>-over-version" may be helpful. This would fetch/install e.g. the <filetype> package version 1.1.3 instead of 1.2.1 which is only available as egg.