UCLA Law's Eugene Volokh explains in a long post why the popular Volokh Conspiracy group blog does not allow visitors to post comments. Excerpts:
1. The first is esthetic, which sounds frivolous, but esthetics of a certain sort matters a lot to writers and editors. I feel that The Conspiracy is a coherent product that I help put together...I have a very high opinion of my cobloggers, and have tried to select them based on their quality. It would annoy me a lot if this coherent product also included some postings that I very much dislike, from people whom I never explicitly invited...2. The second is reputational. Rightly or wrongly, consciously or not, some people's perception of the blog and its bloggers will be molded by what the commenters post as well as by what the bloggers post. Some people will infer (not implausibly) that because (A) some dreck is posted, (B) I have the power to delete it, and (C) I don't delete it, therefore (D) I must agree with it or at least not entirely disagree with it...
The correct answer, at least here, is (E). I often don't agree with the tone and substance of comments posted here by visitors, but won't delete them for that reason alone. Other than spam I think I've deleted two posts and a paragraph in two others that were potentially libelous or anonymously vile. My ethos is that comments posted by visitors are not from me. But I feel Professor Volokh's pain about a site's aesthetic. If I make a change down the road, it will be more about wanting to change the site's tone, and possibly about encouraging more posters to use real names. For now, no changes are planned.