Bloggers tend to dislike deleting comments. This, at least, is the impression I get from various references here and there to how certain bloggers are baaaad because they deleted this or that comment. Everyone acknowledges the necessity of deleting comments once in a while, but whenever the practice is mentioned it tends to be in a negative context.
I think this will change. Think of all the popular blogs with comment sections that you know, and most likely you have blogs with terrible comments. Without naming names, I think we have all seen threads with a few hundred comments degenerate into little more than noise.
In a blog like this where the number of comments is small I can always reply to each comment that I consider unhelpful. This, though, is impossible in large blogs. In that case, either you let the comments degenerate into knee-jerk responses on both sides, petty accusations between the participants, and unhelpful arguments - or you turn off comments. With time, I expect option three - the deletion of comments the author feels does not contribute to the debate - to get much more popular.