When you have free and open debate, even of bad ideas, the best and truest ideas tend to win out over time. Not every time and in every moment, but by and large. As humans, we are all reasoning animals, albeit very imperfect ones.
But when you shut down and shut out bad ideas—rather than respond to them with better counter-arguments—they only fester and grow in the darkness. Free speech matters. Even (and perhaps especially) for those people we disagree with.