Comment 1 for bug 85604

Yup, extras are not always conserved by zc.buildout.
I see at least one place which it occurs, In zc.buildout.easy_install._constrain.Installer._constrain().

As i use personnaly minitage.recipe:egg in flavor of zc.recipe.egg, i work arounded that in this recipe.
You can see the result there (_constrain()):
http://git.minitage.org/git/minitage/eggs/minitage.recipe/tree/src/minitage/recipe/egg/__init__.py