Because there is such a thing as a bad question
It's one thing I've learned watching the progress of the MSDN forums over the last year+... there are bad questions. I'll be watching to see how well this sort of "rate the question" system goes over.
Something new has appeared at Yahoo! Answers . A rating toolbar for each question and for each answer. These provide the ability to quickly click on a "thumbs up/down" to rate each question/answer. There's a button to "report" the question/answer for abuse, another to email the question to a friend and another to add it to your "watch" list, MyWeb, de.licio.us, MyYahoo! or any other RSS reader.