A few days ago, Google introduced a new tag supposed to combat comment spam : adding rel="nofollow" to a link will prevent Googlebot from following it, and thus from giving the linked site any Page Ranking credit.
The idea behind this is : spammers are spamming to get higher Page Ranking from spammed blogs, this tag will prevent any PR increase, so spammers will soon stop spamming. Welcome in utopia.
Spammers are spamming because posting a million comments costs nothing, and still can generate a few clicks on their sites. So why the hell should they stop spamming ? They just won't.
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Anti spam tag rel='nofollow'
Anti spam tag rel="nofollow" : useless.
I agree, also if you are adding this attribute you are devaluing the value of posting for a non-spam user. Therefore less chance of them posting means your blog fails aswell..