I would beg to differ with that generalization. First of all, many bloggers have begun to shun the "nofollow" tag. For whatever their reasoning, more and more bloggers are letting commenters receive the juice for taking the time to participate in the conversation.
Secondly, Google is not the only search engine. While the backlinks may, or may not, be recognized by Google, they most surely are recognized by Yahoo. A backlink check in Yahoo on my personal blog list hundreds of links from comments I have made on other blogs.
Finally, and most importantly, you can use commenting on other's blogs as a source of traffic in itself. Forget about the link juice you may or may not receive from the comment. If you post an insightful comment, the other readers of the blog will surely click your link to see what else you have to say. My top referrers on a daily basis are from other blogs that I have commented on. The reason you want the link juice is to help you improve your ranking, which theoretically will improve your traffic. Commenting can bring you the traffic - without waiting to see how you sit in the SERPs.
For the above reasons, I would say that commenting on blogs is hardly "a waste of time".