Why Is rel="nofollow" important?
There's a debate going on about how effective the rel="nofollow" solution really is. Some believe that Google is flattering itself by thinking that the primary motivation for comment spam is Google juice.
I do not believe that rel="nofollow" will stop comment spam as I've stated before (though I've applied it myself). Getting their links out there may be motivation enough. However I think it will have a bigger impact than some people think.
In terms of sheer economies of scale, I don't think comment spam is like regular email spam. Sending an email to millions of users is very easy both technically and costwise. Buy a list and start sending. Comment spamming millions of websites takes a lot more work.
The primary benefit to comment spam was that a sites page rank increased. Think of it this way, if I spam 100 sites, that might increase my exposure a little, say 10,000 people come via those sites. But suppose those 100 sites increases my page rank and now scores of users are finding my site via Google. That's where the big financial impact comes in. Those 100 sites might now bring in 1,000,000 users. (I'm pulling these numbers out of my ass but I'm probably not too far out there).
Removing the Google juice increases the comment spammer's work by a factor of 10 or even 100 to get the same financial impact. This might not stop the comment spammer, but it will hopefully put a damper on their activities in the long run.