As an SEO company, our goal is to increase the rank of the websites we handle on major search engines. One of the factors is link building. Because of increasing link spammers through comments on blogs and forum postings, most websites already include a nofollow tag. Adding a link on a site with nofollow does not help in increasing your website’s credibility but we are contemplating because it still brings traffic to the site (depending on how interesting your comments are).
We did a little bit of research on this and found out that Google and Yahoo actually still indexes the sites linked from a website with nofollow. You can read the rest from here.