Google wrote a blog post named Hard facts about comment spam. I'll sum it up for you quickly. If you do not protect your site against comment spam, Google will trust your site less and it can ultimately hurt your rankings.
Most blogs these days have automatic comment spam solutions, but you need to make sure your blog is handling it correctly. I check comment spam here several times a day. Although most gets caught, some leaks through and I need to take care of them manually. Yes, I have them all nofollowed anyway, but comment spam looks bad and it distracts from the content on this site. I am more concerned with it distracting from the content here then Google trusting me less - too be honest. But most bloggers are more concerned with Google trust over reader trust, so either way, this is something you need to take care of.
How does this hurt your rankings? Well, if Google trusts the links on your site less, then your internal linking power dwindles down and you rank lower.
Google offers these tips:
- Disallow anonymous posting.
- Use CAPTCHAs and other methods to prevent automated comment spamming.
- Turn on comment moderation.
- Use the "nofollow" attribute for links in the comment field.
- Disallow hyperlinks in comments.
- Block comment pages using robots.txt or meta tags.
There is a lot of discussion around this blog post at Google. Some seem surprised by this, while others do not.