About 10 days ago, Google published a post about comment spam on their WebmasterCentral blog. They have warned that they have the means for understanding the link graph & they can determine and tackle comment spam links using algorithmic methods. They can even determine the fine-line spams that some webmasters deploy like instead of blunt spam they will say “great site!” as a comment with a keyword as their name. So in best case, you will receive a little or no SEO value at all for posting meaningless comments in other websites & blogs.
But still I doubt if spammers will stop spamming. These may be hard facts about comment spam, but comment spammers are hard nuts to crack. They perhaps just like to spam