A comment I just wrote on the ProBlogger blog:
"most comment links are no-follow anyway"
I would not dismiss no-follow links or their usefulness that easily. While they may not carry the same weight as a "follow" link, SE bots do spider no-follow links and any popular website will automatically get a decent number of "naturally acquired" no-follow links for every "naturally acquired" do-follow link it gets.
Several social media platforms [add] no-follow [tags to] links, so when a site becomes popular and people start talking and tweeting about it, it will get lots of no-follow links. One could even argue that a big surge in do-follow links that is not accompanied by a similar surge in no-follow links will be suspicious as it could be an indicator of a big link buying campaign.