Marco and Dan spent a good long time talking about blog comments on this week's Build and Analyze (starting at 64:50, since I don't know of a way to deep link). Marco summarizes this way:
I think comments solve a problem that a lot of people don't have anymore, in a way that encourages a lot of bad behavior. And we have much better solutions to these problems, now, that have far fewer downsides. And comments are these relic of the past…
I'd add that the past that Marco was talking about was back when coming up with a blog was much harder. It required having a host and installing or building a CMS, maintaining it, and adding content. Comments back then were the only way to get validation and feedback on your writing. Now that anyone can have a blog in just a few minutes of typing with tumblr or Squarespace or any of dozens of options. By doing this, you have an immediate commenting system for literally anything on the Internet.