why should you stop commenting on blogs

Many site owners use Spam tactics by creating bots that crawls around the web looking for open forums and blogs  where its easy to add comment  and get a backlink to their website , which leaves behind automated comments bordering on spam, which are most of the times, not related to the content of the site in question
The great majority of these sites are deleted  by the  rel=NoFollow  tag or by the blogs software content management systems. however the spammers do not care as they operate on a huge scale

Reason number 2 why you should avoid comments across blogs and forums is most of the times, the comments do not have any relevance to the topic. If you do have to comment ,ensure you stick on the context and relevance, this way. For Example sites like quora and a  the seo forum  like webmastertools, has user generated content which is editorially very sound.While blog and forum site owners might use rel=NoFollow tag, as we saw in the previous post, Search engines can decide the quality of the post and the site which is linked too, while the no follow tag might be used, it only stops passing of the link juice to the page linked to and not the indexing

Link Farms  and Number of links: due to the nature of the blogs and forums, just anyone can leave behind a comment, irrespective of relevance or being topical. Over a number of time Forums and Blogs  acquire a huge number of links, with search engines viewing the links as not relevant or even ignoring  the links .Worse a webmaster that has many related sites , might ask a link back in exchange for 10 other sites he owns.  This way all links are interlinked and your site might be a part of the "link farms" which google penalises very heavily