Ignoring ideology and human rights and focusing solely on practical effects there's 2 sides:
1. people who are already radicalized should have some place to talk so we can know what they are talking about and so they are more likely to talk instead of acting
2. people who can be radicalized - are, because there's places where they can encounter such ideas
I've never seen any scientific study estimating which effect is more important. From ideological POV I'm in favor of more free speech when in doubt.
What I neglected to say in my original comment was that the political camps tend to exaggerate the "dangerous rhetoric" of their opponents and downplay their own. This is especially true when the statements require some specific interpretation in order to be treated that way, since their plan spoken version is not in itself dangerous.
1. people who are already radicalized should have some place to talk so we can know what they are talking about and so they are more likely to talk instead of acting
2. people who can be radicalized - are, because there's places where they can encounter such ideas
I've never seen any scientific study estimating which effect is more important. From ideological POV I'm in favor of more free speech when in doubt.