Friday, August 21
Technically, dofollow does not exist. It is a shorthand for no nofollow. rel="nofollow" for individual links was invented by Google in 2005. Other search engines followed suit. The intent was to let search engine know what links you do not vouch for.
The ensuing years brought the term "page rank sculpting" to life. Webmasters thought they could sculpt google page rank based on where they place rel="nofollow".
In 2009, Google decided to change the way they distribute PR, or "link juice". They would still not give any link/PR juice to nofollow links as usual, but they would reduce the link juice of ref="follow" links by the amount taken by the rel="nofollow" links on the same page.
How does this affect dofollow? Nicely. The dofollow links make sense now in the comments, as they take away as much link juice from the post links, as rel="nofollow" links. So implementing dofollow links has no negative impact on your blog's link juice, but it has positive impact on your blog commenters.
You need to implement good anti-spam methods in your blog, including captcha and moderation. rel="nofollow" is not a tool to reduce spam, so go dofollow.
Google still (8/2009) believes rel="nofollow" is a spam deterrent:
however that may change soon when more bloggers get a hold of effective and easy-to-use blog spam prevention tools.