An interesting thing about the most vocal defenders of free speech at all costs is that they often conflate free speech and rational debate. Which is a strange thing to do - if you argue something with loudness and extreme forwardness, the least that could be expected from you is that you know what you are on about. Yet, somehow, free speech maximalists often show a brutal lack of understanding of the difference between rational debate and free speech.
To illustrate the difference, I shall describe a case where it is not rational to engage in public debate, and where the debate itself has detrimental effects to the society within which it takes place. The debate in question is whether it is the right course of action to exterminate a specific group of people.
For those who belong to this specific group, it is not rational to participate in such debates. The most immediate reason is that you might lose. No matter how unlikely, the mere possibility of losing is reason enough to stay clear of such debates. To the proponents of the extermination policy, your participation in the debate is an additional justification for their point of view. "They can't even defend themselves!" they'd claim, and then move from word to action. Perhaps not immediately, but eventually the final day would come.
The tragic part is that you would lose even if you won. If you won, it would most likely be because you gave reasons for why your extermination is a bad idea. These reasons might be good in and of themselves, but there would be a finite amount of them, and with enough journalistic efficiency these reasons could be summarized into a list. From the very moment the debate ended, this list would constitute the reasons society abstains from exterminating you.
The existence of such a list would constitute an opening for those who favor your extermination. One by one, the proponents could work to undermine these reasons, until they are no longer seen as sufficient reasons for abstaining. The debate would reopen, and you would find yourself in a weaker position than last time around. You would yet again have to defend your right to exist, and you would have to do it using an ever shrinking range of possible arguments in your favor.
Needless to say, this process would continue until there are no reasons left. And then the proponents of your extermination would have won.
This is detrimental not only to the group targeted for extermination, but also for the society as a whole. For each round of these debates, the society would slip one step closer to enacting genocidal policies. Which, to any decent and moral person, is not a desirable outcome.
The rational thing to do in order to avoid such an outcome is to simply not have these debates. Exorcise them public discourse, and keep them off the realms of possible topics. Do not entertain the thoughts, shun those who persist in proposing them, ban them from polite conversation. Keep the opinion marginalized. No good outcome can come from having these debates, and thus the rational thing to do is to simply not have them.
Free speech maximalists want to have these debates anyway, in the name of free speech. But they conflate free speech with rational debate, and as you have seen, there is a very concrete case where these two things are mutually exclusive. If they are to be honest to themselves, they will eventually have to make a choice between one or the other.
If you began reading this post with the opinion that we should have these debates anyway, and still hold that opinion, then I want you to be fully aware of what you are proposing. I fully trust that you will, in your own time and on your own terms, make the rational choice.