On November 27th, 2009, Google has written facts about spam comments, its explain the influence of spam comments on your website ranking and whether is it useful to your site ranking or not.
We would often seen a comments that contain links to the comment author's site, or generic comments like "Nice info", "I like your site" with a commercial user name or keyword linking to their site. Come on, it's look too spammy and you must know that!
I took some points from that article why spam comments is bad and useless:
For those who do, it is actions that worsen the quality of the web, comment section should be a place to share opinions, or to add additional information to the article itself, and not a place to list of a spammy keywords.
If it is done only for the pursuit of ranking then you do the wrong thing. If your site has an original and useful content then the ranking will go up by itself.
You can read the article from Google about spam comments here.