Blogs should be overlooked. Try posting a link. Press view--> source. Check the link 99 times out of a 100 its got rel="nofollow" attribute.
OK, most don't give PR, but some do, it depends on how they were set up. But remember that the main idea, the "natural" non "seo" reason for a backlink in a comment is to lead visitors to the website you're linking to, whether yours or not. If the site is worth it, among all the "readers only" there are some that are also webmasters and might want to put a link to your site in theirs. Now that would be the most natural and therefore precious way of getting a backlink. Often from a topic related site. In other words you wouldn't get many, but the few would be very high quality links. Quality vs Quantity, what do you think? Of course if your website is rubbish then you're right, it would be almost a complete waste of time.
http://mypagerank.net if you will place their script to your code, they will link to you from PR6 site