"Comments are content. Why google or any other search engine would want to ignore such content is beyond me. Needless to say, google and every other search engine would want to filter comments just as they do other forms of web content for relevancy, but too simply ignore such content seems to be counterproductive."
Well if you read Google's original posting, I see that the nofollow tag, basically means that Google's index bot will not follow a link posted on a comment section of a blog. Thus such linked websites will not gain from the link popularity of the linking blog, where the comment has been spammed. This is good if the comment is a spam, but not if the comment was valuable and may lead to another valuable website. So google will index the content on the comment, but will not follow the links to other websites. Google might lose out on valuable content (maybe) and such websites will lose in the page ranking game.