Well if someone came to me with such an argument I would say it's silly to judge open source software by the long tail of least-successful projects. I mean no one chooses anything by focusing on the worst representatives of a category. People aren't evaluating the least-maintained open-source software any more than they are evaluating all the dead and gone software to be found in bargain basement bins.

There's plenty of great free, open-source, and proprietary software out there, and many interesting debates to be had about the merits of competing packages. If some pointy-hair insisted on an obtuse argument about how all the crap projects on SourceForge are diluting the power of open source, I would get another job and let him replace me with someone with a higher tolerance for incompetence.