Yes, it is very much useful in seo
As you can see that when somebody is searching for answers yahoo answers come in top 3 spots in google search, and that means high probability of your link to be seen and clicked by user !
I have read alot of bad advice to people over the time because of websites like yahoo answers and youtube having no-follow link.
I can see very clearly in my backlinks in google webmaster tools that I have generated backlinks from yahoo answers and not only that I have incoming traffic to my website that are interested in my website and thats things search engines are looking at these days. (Traffic to a site and page views)
So it is definately yes, don't believe anything else you read
Recently I researched and read alot about yahoo answers, youtube and all no-follow links and spent alot of time reading. So I'll break it down quick and easy...
Posting in yahoo answers is definately good for off-page seo but not in the normal backlinking sense. Most off-page seo is about dofollow backlink popularity and relevance but... a site without any nofollow links is obviously trying to cheat the system and search engines are getting more complex (especially google).
Building nofollow links builds trust with SE's as you are showing the links are being built naturally with both do follow and no follow.
Seriously how many no follow links do you think youtube has....
My Final Say: Stop discussing and debating whether nofollow links help you with seo or not, a site with only dofollow link is obviously not building links naturally and resources like twitter, youtube, facebook and yahoo answer are great places to promote your site and generate traffic, whether they help your seo directly or indirectly they are still helping your site....
[quoute from pingler]While in the past nofollow links were completely ignored by search engines and their ranking systems, the changes in how people interact and share content has changed drastically in the past few years thanks to many platforms such as Twitter and Facebook, which also provide nofollow links but are still read and ranked by search engines. We will get to the bottom of this question and determine if it is worth your time to worry about nofollow links.[end qoute]
There is another good post on seomoz