Yahoo Answer is pretty targeted. So when you use this services don't expect the backlink but more focus on the traffic you get. make sure your site has pretty good content and usually they will response well. and I have found that is better you put your link just in the profile. putting your link on the resource box or answer will later be reported as spam by the other users.
I love using Yahoo answers, If you write good answers you will get a response, especially if it is great topic.
Yahoo Answers is a good place to acquire free nofollow one way links. Currently, Google gives backlink credits for any of the nofollow links from Yahoo Answers. By the way, the targeted traffic from Yahoo Answers is also free. Do the SEO today and go answer some questions.
Answering in Yahoo will help you to get quick index in Yahoo search engine. One of my site got good results from Yahoo. But not in Google and also no effect on PR also!
Google gives a lot of importance to Yahoo answers in SERP Results Yahoo answers are important from traffic point of view