Commenting on any blog is one of those ways used by many foolish bloggers to increase visitors and get some backlinks. Actually, it is the most worst way to get visitors and get backlinks, because there are many reason behind it and that is what being discussed in this post have a look.
rel='nofollow' attribute on links in comments
Your comment is bieng deleted by author or marked as spam
Those bloggers who want to increase number of comments and remove links, Disqus commenting system - that I know maybe there are more - let you edit the comment of anyone (if you are comment moderator) and you can easily remove the link from that.