But PR does have one purpose, it keeps a whole army of bloggers in business speculating about the Google algo for it.
I've also heard rumours that Yahoo are going to put Trust Rank on their toolbar, though there's so many rumours in the SEO community you never know what or who to believe. I've found that it's best to take most of the speculation with a large pinch of salt.
I've never noticed a change in traffic or SERPs when the PR of a site has changed, the only difference is that the number of link exchange requests increases exponentially with every PR point you gain.