Yahoo Answers worth for link building?

In webmaster forums like DigitalPoint, GetAFreelancer, Elance, NetBuilders, V7N and WickedFire, only a few webmasters are offering a job for us which is Yahoo Answers posting services. But why is it that Yahoo Answers set the links to no-follow? It's because Yahoo Answers are not really designed for SEO purposes, but in order to help people in answering their questions for such pride. But why would they do that? Just for backlinks? Many people or user accounts got suspended because of spamming and advertising their links on every answer they're involved. You have to be wise when you are answering something, and the source you are promoting. Webmasters pay you for such good quality in answering questions in Yahoo Answers.