Not really good or effective for PR but if the site is reputable (like Yahoo! answers), you might expect little traffic.
NoFollow backlink are that which can not be considered by the Google for counting your PR. This is the way by which several sites are avoiding the spaming from them. For that they have one attributes in their link tag and that is : rel="nofollow" so Google can get that this link is not considering as Backlink.
At the very least they're good for link diversity. And if you're generating them from sources such as Yahoo Answers, forums, etc, you can expect to drive some traffic if you're leaving good replies.
They're not really going to help your PR, no. However, as JSProjects mentioned, they might help you create more traffic to your site. A site like Yahoo! Answers gets a lot of unique visits every day, so if you can divert a little of their traffic to your site, you'll profit.
Nofollow backlinks aren't as good as dofollow but they do still help, there is a good article here on how to use nofollow blog comments as effectively as possible - http://3finking.blogspot.com
I agree with you. Anyway, you just also need to be sincere and responsible in your comments for the benefit of the readers.
NoFollow backlinks are that which Google can not consider while counting the PR of your website. So you have totally loss for generating that link. But it is true you can get the traffic from that.