It’s true that some folks still do not see spam as all that big of a threat. After all, it’s not really hurting anything, right? Wrong. The damage from comment spam is something that can if left unchecked, could even effect your site’s revenue and ranking.
 
According to this piece, comment spam not dealt with could actually affect your website’s ranking. Why is it such a big deal? Because you could be seen as allowing other sites to use yours to create problems with black hat SEO. It’s certainly not a good thing.

Luckily dealing with this problem is not really that big of a chore. Any decent CMS provides tools for stamping it out. The single best approach is to NEVER allow posting to your blog