Some bloggers have recently discussed their aversion to having comments enabled. While I think that Scheherazade did an excellent job of defending comments, I feel compelled to add a few things I've found valuable about having them.
1. People are much more likely to leave a comment than to write an email. Email is more formal, non-anonymous, and does not leave a public record of protest. Unlike Jeremy, I'm not concerned about being upstaged by commenters. I find that one of the best aspects of this reduced cost of communication is that people are more likely to correct me. I hate looking stupid and not knowing it, so I appreciate readers who let me know when I went too far or made a mistake. We all learn something, the blog is improved, and everyone wins.
2. I can count the number of strangers who have emailed me about this blog on one hand, but have discovered at least a dozen new blogs by following links to commenters' websites. Because they read my blog, we often have some interests in common, or at least more so than random surfing produces. Comments provide me with additional web entertainment!
3. Comments are not always from strangers. I have several classmates who read and comment semi-regularly, and since we no longer live in the same apartment (or even the same coast), it's a convenient way to have a conversation with old friends. Sometimes surprise guests even appear - like Christiana or Ray - and we can all circle around and give them a virtual hug and ask them why it's been so long.
4. Finally, reading comments (or their more conventional counterpart, letters to the editor) on any website functions as a rough gauge as to when a website has jumped the shark. Once I find the comments or letters more informative, well-written, and interesting than the post or article they address, it's time to go. There are some exceptions to this; a site can integrate comments with its format such that the site is largely discussion based from the beginning. This, however, requires a certain humble approach to blogging, with an emphasis on creating posts that provoke thoughtful replies. (Update: Teresa Nielsen Hayden, whose website sets the gold standard for valuable comment threads, comments below that the best approach may not be to keep the riff-raff out but to guide the discussion by participating. I defer to her experience.)
Will may bemoan the signal to noise ratio in the comments section of popular blogs, but for those of us who are in the less traveled areas of the blogosphere they have their incidental benefits.