Traffic and a balanced link profile. Pursuing links that are only "dofollow" is not constructive to that end.
Plus of course no-one is really sure anyway on how nollow links are assessed by Google i.e. we know they do not pass PageRank but do not not know what else of value they may have.
No follow gives you traffic and it is required to keep a balance between do follow and no follow. Do follow helps you to increase your in bound links and ranking where as no follow helps to enhance the traffic to your site.
The first five replies have all mentioned traffic, so I think we can say we are aware of that benefit now.
Please post if you know of any benefits other than traffic and having a balanced link profile.
The easier it is to get a link, the lower the value. However, the more your site is talked about across the net, even if the links are "no follow," the better. Exactly what Google "knows," no one can say with certainty.
However, one must balance that concept with webspam. Is forum activity only being done for the link? One should always add value and never do things only for the link. Doing so is spam and Google is on a campaign to wipe it out.
Almost high PR and rank sites are nofollow links. I'm not sure that nofollow have no impact on PR and rank.
So in my opinion we should have both nofollow and dofollow links, nofollow links are still counted as backlinks