In all my posts about SEO I've written about the importance of writing for people, not the search engines. My point being that what you actually want is for people to read your site, or blog, and to like what they read, because people will buy from you - the search engines won't.
I saw a great post from Jack Humphrey today which was actually on the question of the 'nofollow' flag and how the hullabaloo over 'nofollow' over the past year has led to a general decline in the amount of comments being left on blogs.
Jack's point is that to avoid leaving comments on blogs simply because they have the no follow flag set is costing you traffic and, in any case, is the wrong reason to decide not to leave comments.
You're making a decision on whether to comment based on what the search engines will make of it. Which is no different from writing for the search engines
Comments are an essential element of the blogosphere and, I have to admit, one that I have sorely neglected. Undoubtedly to my loss. But they are an essential part of it, and vital to building the flow of knowledge.
To avoid commenting on a blog because it has the no follow flag set is hurting both you and the blogosphere.
Here's the post.
And here's my commitment to getting my ass into gear and being more participative (meaning I'm going to comment more) in the future.