It being pulled in automatically by twisted is the point.  Also, if some new twisted version comes along that we upgrade to, and it requires a newer zope.interface than we specify because we didn't notice they changed it, then suddenly we have install problems (unless pip can resolve those somehow).
In my install tests I haven't noticed it taking much longer.  zope.interface certainly doesn't take very long to install (at least for me); much larger libraries (twisted, pyopenssl) take up the bulk of the install time.

Possibly the "your system isn't the only system" argument applies here, but I don't think it's _that_ big of a problem.