> "Doug Walton, Canadian academic and author, has argued that ad hominem reasoning is not always fallacious, and that in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue,[10] as when it directly involves hypocrisy, or actions contradicting the subject's words. The philosopher Charles Taylor has argued that ad hominem reasoning is essential to understanding certain moral issues, and contrasts this sort of reasoning with the apodictic reasoning of philosophical naturalism.[11] Olavo de Carvalho, a Brazilian philosopher, has argued that ad hominem reasoning not only has rhetorical, but also logical value. As an example, he cites Karl Marx's idea that only the proletariat has an objective view of history. If that were to be taken rigorously, an ad hominem argument would effectively render Marx's general theory as incoherent: as Marx was not a proletarian, his own view of history couldn't be objective."

(wikipedia article on ad hominem)

In this case, i'd think it's pretty fucking valid.