There is code in buildout to remove develop-eggs that it knows about.
So if everything is OK, develop-egg removal works fine.
If there's something fishy goign on, however, buildout
doesn't clean it up enough. Zapping the entire directory upon bootstrap
is a very effective way to prevent problems.

Reason: the old osc.recipe.sysegg did add develop-eggs .egg-link files to the
site-packages dir, effectively short-circuiting buildout's picked versions.
Likewise an old bootstrap could have left a setuptoos.egg-link to an ancient
setuptools version. I just this minute had to help a colleague with just such
a problem.