Jeremy Zawodny revived the discussion about the actual usefulness of rel="nofollow" mechanism in hyperlinking. In short, it does not prevent comment spams. Moreover, it hinders visitors from commenting as there lacks "reward". Jeremy concluded, which I completely agree.
Look. Linking is part of what makes the web work. If you're actually concerned about every link you make being counted in some global database of site endorsements, you're probably over-thinking just a bit. Life's too short for that, ya know? Link and be linked to. Let the search engines sort it out.
Yes. Just let links do what they ought to do, and let search engines do what they ought to do.
Currently there are two camps -- those who absolutely hated it and those who tried to get everyone to endorse it. Personally I do not approve nofollow tags -- not to a point of hate but just could not understand why we need this. Besides they have achieved absolutely nothing in stopping the comment spam, they add extra overhead for web masters and bloggers to worry about.
However one thing that disgusts me the most is how some people used
rel="nofollow" to explicitly discount or disapprove the link. And they make sure others know about it by adding "(nofollow used)" in bold type after the link. It is really search engine's job to work out whether a link is public or negative publicity.
As of comment spams, they are probably dispatched by thousands of zombie hosts on the Internet. They only have one goal -- to get the visitor to come to their site. They don't care whether the traffic is by people clicking on a link in the comment spam, or organically from the search engine results. But do they care about the SERP ranking? I am not entirely sure about this one. However if people are looking for that those "specific" product, be that phentermine or Texas Hold'em, they will not be able to compete against those information sites anyway on SERP, as search engines are actually doing their jobs to actively discounting spam sites.
At the end, what eliminates comment spams? Search engines that can distinguish between spam and legitimate sites, and good well-trained spam filters that can distinguish between spam and legitimate comments. Definitely not