For me the ability to comment on other peoples blogs is a privilege which greatly enhances the whole blog reading experience. If you write something which pulls a string with me I like being able to tell you with a quick comment.
I am aware however that there are limitations. In particular it seems to me that commenting works better for less popular blogs with a small numbers of readers. If you leave a comment on my blog for example I will be delighted and in reply I will certainly read it and probably reply unless you are advertising WOW gold (mind you in quiet weeks I have even been known to reply to spam bots). I have pretty much given up commenting on the big commercial blogs though like Rock Paper Shotgun. Who on earth reads all 119 comments to a post like this. I certainly don't and I doubt the authors do either.
Even popular non commercial blogs have way too many comments to read. Tobold's posts often attract 100+ comments sometimes involving heated discussions. Or so I believe. Let me tell you a dirty little secret. I don't read the comments to Tobolds posts.
You see blogging is not a public discussion forum. It is a personal diary made public. Commenting on a blog is a public reply to the author of the blog. The only person you should expect to read your comment is the blog author himself or herself. Third party discussions really have no place in blog comments. I think that a some people forget this.
Tobold has cited the the the level of abuse and antagonism from a vocal minority of commenters as one of the main reasons he decided to take his current break from blogging. I do hope he returns and I hope that he doesn't take the decision to turn off comments.
Bill Harris does not allow comments on his blog Dubious Quality. I wrote to him about it and he explained that he consciously took the decision to avoid hassle. It's a shame. For me a blog without comments isn't really a blog. Every time something he writes catches my eye I get frustrated by the inability to comment and swear to knock Bill off my reading list but the quality of his stuff keeps me reading ... so far.