Back in the day, there was a major problem with people posting garbage comments on blogs or forms so they could get back links to there site. Every time they made a new post they would get anther back link and there ranking would rise.
Then google and yahoo introduced the nofollow link parameter that tells a search engine not to add rank to link in a comments section. Over night all the sites that have been posting garbage on blogs and forms dropped out of the ranking. The level of garbage comments dropped significantly but so did the lagitamit comments too. Since people where not getting any benefit from making a comment people stoped taking the time to leave them.
Then someone came up with the Dofollow WordPress plugin that removed the Nofollow parameter from links in the comment section of sites, presumably to get more people to comment. Fight spam not blogs
This would have worked fine if only a small number of people used this plugin and no one knew who used it. But they created a list of all the people using the plugin. This list is a spammers paradise, 1000s of blogs with out the NoFollow parameter. They can make a useless comment on each and every one to get a back link and there ranking will rise.
Also the context value of outgoing links in your posts would drop because there would be so many outgoing links. In other words the value that search engines would give your links that you made in your post that people are commenting about would drop.
The follow plugin seems like a good idea, rewarding people that make good comments. But having a list of people that use the plugin is just asking for trouble.
With this post I can go thou the list of people using the follow plugin and create a comment with a link back to this post, increasing my ranking.
I found out about this plugin from image promoting the Dofollow plugin on jonlee.ca
Edit While looking around for people to spam this post on I found a post by randaclay.com that talks about a plugin that only removes the no follow link for trackbacks. This is a much better idea, for one it rewards the people that talk about and link to your content. Instead of people who make spam comments.