I think it's about tradeoffs, which is ultimately a judgment call.  I think there is certainly value knowing that the docs build, and integrating that into Travis tells us every time if something breaks the docs.

But the cost incurred with compiling matplotlib for a small part of the docs should not be ignored.  I agree with you on the warnings part, but *documentation building warnings* are much less an issue than *warnings related to distarray itself*.

For me, a few sphinx warnings aren't an issue, so I'm in favor of removing matplotlib.  Others?