A lot of people conflate the morality of the author with how important or 'correct' they are. You don't really have to like or even agree with Foucault to admit that his writings were important. I personally think his takes on Marxism were wack as shit, and that he didn't have a serious political project, but there's some value in his "archeological" approach and how he thought about things like power and subjectification.

Edit: There's no better example of how even someone who is both an asshole and wrong can still have some value to his writings than Adam Smith. Marx probably wouldn't have been able to write Capital without reading that bourgeois fuck.