There are several reasons to allow comments in your blog, but the one that speaks most to me personally is learning from people who are smarter/wiser/more knowledgeable on a given topic than you are.<p>If you're blessed with readers who're able - and might be interested - to challenge you, call you on your bullshit, point out your faulty logic, tell you where you can learn more, and explain what it is that you don't know you don't know - then it's crazy to intentionally disallow comments, as far as I'm concerned. Sure, sometimes you'd get feedback by email, but mostly you won't. Looking at myself, I'm about 10 times more likely to leave a comment on a blog where I can add nontrivial information, elucidate a point or give a reference, than send an email to the owner. Are other people very different?<p>To me, a conversation with knowledgeable and intelligent readers is absolutely worth ten times its weight in fluffy or incoherent comments. Of course, there're people with different priorities. There're plenty of people out there whose blogs are essentially PR vehicles. To them, having comments is probably only worth it in terms of drawing more readers to the site; and that can be offset by other reasons. (I just guessed, without looking, that Seth Godin doesn't have comments in his blog; then I went to look - turns out I'm somewhat wrong; most entries are closed to comments, but there are rare exceptions).<p>I get a huge number of comments, because I write a popular blog, and many of them aren't worth 
my or anyone's time. I learn so much from the other kind, however, that it's absolutely worth it to wade through all the fluff. Every time I forcefully present a point of view I hope for someone to intelligently challenge it, and most days that's what happens. The value of that is difficult to overestimate.