A whole ton of low quality backlinks especially unrelatted to the niche make no sense at all. I have hundreds of them but with very little impact if any!!
First off, PR has nothing to do with SEO. This thread should be moved to the official PR thread in the Google forum. Second, trying to gain toolbat PR is a waste of time. The SERPs ARE NOT RANKED BY PR, so it will do nothing for traffic. Of coarse I am assuming you want traffic, not just more green in the bar because it looks pretty.
Do link building , u can buy also paid links, make article submissions, directory submissions, forum posting, comments posting, bookmarking, u can say SMO.
But it is not worthless. PR is like a level or how much does Big G trust your website. So it is much more than just a green bar. But as was said earlier on this forum "Webmasters are the only ppl who see that small green bar". But high PR can help you a lot in link building. So... it has something to do with SEO >.<
Always try to comment on relevant blogs same as your niche is. If it is dofollow then it's an added advantage..