I'm tutoring someone in Philosophy, focusing in free speech. It's interesting to think about how you would defend an idea so absolutely fundamental to Western liberal society. Former Supreme Court Justice Holmes put the problem as a cute paradox, which I will paraphrase as
My gut tells me there's an analogy to freedom of religion. These two freedoms are both interesting in that they're not regulations of citizens but rather regulations of the government. We forbid the government from forbidding religions or speech. It's actually historically interesting that, in fact, all of the Bill of Rights in America are of this form.
But I have in mind a deeper equivalence between freedom of speech and freedom of religion. It seems to me these freedoms keeps people from warring with each other through the government. Even if one religion is wrong, and the vast majority of citizens agree that it's wrong, we still refuse to persecute followers of that religion. Even if the religion is harmful to its followers and others, we refuse to regulate followers' speech and beliefs. One reason for this is that, in societies where people try to control the religions of others, we have always seen brutality and abuse come from people who believe in the righteousness of their beliefs. It may be fine for a while if you're the one who gets to be brutal--maybe it's not fine, but those people think it is anyway, and would not be convinced by sympathy for the oppressed. But I think they should be very concerned for the day, which seems to inevitably come, when they are vulnerable to another group with the same oppressive tactics.
I think the same sort of idea is true for freedom of expression. You might want to regulate Nazi hate speech, but by accepting that the group in power gets to dictate acceptable speech acts, you should worry for the day when people hold power and enforce a set of beliefs you don't like. I think actually the analogy extends to a lot of other topics, like gerrymandering: You might love it or at least tolerate it when your own party benefits, but we should all oppose it on principle, always, for fear of what happens when the other side can use the same principle.
Oddly, I always thought I disagreed with Hobbes, and yet here I am thinking that one good reason for a lot of our rights and freedoms, and constraints on government is a very Hobbesian idea by which we all lay down our arms against each other simultaneously.