It is useless.

Spamming costs almost nothing, if there is a slightest amount of gain in it for the spammers they will keep doing it. Of all those hundred thousands of people visiting blogs and seeing comment spam a few will still follow the link, those few are enough for the spammers.

One could argue that the websites the comment spam point to now haven’t got a really high pagerank anymore. This is only partially true, for only a selective amount of people install the code to add the rel="nofollow" attribute in links that can be spammed. Even though the highest ranking blogs have already installed it, the tons of small blogs are still enough to raise the page rank enourmously.

In my opinion search engines should just ignore links which are considered spam.