My question is, is this generally useful enough for test suites beyond the Python standard library. Essentially it provides a command line interface to specialized test skipping. 

I *still* feel that a generalized mechanism for adding command line options to the standard test runner would allow for this and a myriad of other slightly specialized use cases. It's hard to see how to do that cleanly without full blown extension machinery (which I'm in favour of and have prototyped but it is a *much* bigger change set).