Doesn't seem to deter a large proportion of spammers, either--- I guess spamming is cheap enough that you might as well shotgun it, instead of bothering to research each site's nofollow policies. If you ever look at Slashdot's submission queue, there's a ton of spam in there too, despite the fact that it gets pretty quickly buried, is nofollowed on a robots.txt-prohibited page that not even many humans visit, and has no chance of being posted by the manual story-posting process.