While PR doesn't directly correlate with rankings, it is indeed a measure directly from google on how "strong" a particular site is. Kind of a weak, lagging indicator, as PR is actually calculated in realtime but they batch the updates and push out a snapshot to the toolbar.
I don't go all hardcore and obsess over it, but I do keep track of where my pages go and so forth.. if anything it can tell me if I have a bad ratio of homepage-to-subpage links, which is important post penguin/panda.
That said, my sites did see a rise, so I'm happy. Any positive news in this current SEO climate is good for me!