In a recent Google Webmaster Blog post , a detailed information is given about how comment spamming can lower the rankings of any website in particular. When someone posts a comment that is both irrelevant and posted just to pass the link so as to increase the number of inbound links to their website in particular, the rankings of both the sites will get affected in the process. This is applicable for those comments only that hold no relevance to the post and are completely unnecessary.

Read more: 
Google: Spamming Other Sites Might Lower Down Your Own Site Rankings!