A lot of bloggers have a policy of deleting offensive comments and banning the perps. These bloggers aren't trying to stop anyone from speaking, just declining to host that speech. Perfectly fair. Yet I don't like the policy.
Part of the reason has to do with my early experience on USENET newsgroups. Those groups were pure anarchy. Yes, there were jerks and idiots. Yes, they said some offensive or stupid things that I wished I hadn't wasted my time reading. But sometimes the people who said the most intemperate things also said the most interesting things.
I was often a target of hateful or angry writing. I learned to live with it, developed a thicker skin, and learned to give as good as I got. In fact I learned a little too well, and since then have regretted some of my nastier on-line comments. But we live and learn, and it was an incredible learning experience.
That's why I wish bloggers would be a little more thick-skinned and allow more free-wheeling comments. How do you know what your critics are thinking if you only let your friends talk to you? And isn't it rather important to know what your critics are thinking, even if they don't express it politely?
One blogger I know alternately complains about liberal commenters and the fact that there aren't enough comments. What does he expect? You don't have really good discussions among people who all agree with each other. BORING! If you want lots of comments, you should welcome comments from people who disagree with you. That's what makes the comments section interesting.
There is also a rhetorical reason not to edit comments. When someone from the opposite side of the political debate gets on your site and says nasty things, it actually helps your side of the debate. See this post at Back of the Envelope for a good example. No one makes converts by insulting people and behaving badly. Let the opposition show their true colors, it will only make you look better.