We read what falls under our eye. If I leave bad comments lying around in plaintext, every person who comes along will read them and get a hit of their nastiness, or mendacious headtripping, or whatever's going on in them. "Free speech" doesn't fix that. It doesn't lift the weight off your heart, or clear the crap out of your memory.
I can deal with that. So can you. A lot of people can't. And oddly enough, a lot of those people who can't take the constant headbutting and unpleasantness have more interesting things to say than your average internet rhinoceros.
If you think an individual's right to say anything he wants, in any way he wants to say it, is the most important thing the internet has to offer, all I can say is that Usenet's still there.