On Pharyngula, PZ Myers cites Carl Zimmer as follows:
"People who start a blog and don’t have a commenting function – I just think they’re cowards. I mean, if you’re going to be out there, you’ve got to have a real blog. Everybody else does! It’s kind of pathetic to be a professional journalist and feel like you can’t handle the heat. All those amateurs out there allow comments, and that’s what makes a blog really interesting, because it’s a conversation."
Jim Davila might have a few things to say about this, and "cowardice" may be a harsh judgment, but I agree with that Zimmer is getting at. A blog without a comments section is like a presentation without a Q&A session. Dialogue makes the blogosphere what it is.