PR is good thing for now days, If u get more back link for your blog and get increase traffic of your blog. If link exchange related for your blog post its good thing.
Really PR doesn't matter because there are so many ways to cheat it. You may want to stick with what google says, if the site is relevant and not a link farm=more than 50 outgoing links per page it is a good idea to exchange.
It doesn't matter if the site that you are linking have less pr than your, which is really important is the relativity of that site. And who knows maybe one day that site get higher pr than yours.
It does not matter! Now a days backlink from very high PR site hardly matters. You need to have more and more backlinks. offcourse not from bad quality blogs and other sites, but yes quality backlinks will help you a lot. Yes you can get backlink from no PR blog for your PR 3 Blog there is no harm in it. Go on and have it.
As long as the site that gives you backlinks has PR value in it, it'll pass some PR value to your site.