I have fond my answer many times with the help of yahoo answers, just by typing on google, i have never registered for this program, i also heard that they are very strict. They ban you if you don't give a good answer or something like that.
Yahoo answers is sometimes interesting, but I don't see the potential for seo. Maybe slight traffic, but not too much unless it is some odd ball viral internet thing. Which you never now. As far as I check most everything there is nofollow.
When I have something which I do not understand, I often search Google. If Google does not give me information which I want, Yahoo answer is my best solution. Actually, many people will give me information which I am looking for. Yahoo answer is really great.
yahoo answer is still good.. you can still get sensible and helpful answers .. it is also helpful with the seo for it help drive good traffic. posting answers relevant to the question and at the same time putting your link to it.