Anyhoo, obvious sense is obvious sense. PageRank, and it’s weight in overall page ranking for specific queries, is near moot. (Okay, maybe ‘moot’ is a bit strong, but you get the picture, eh?) PageRank is a direct measurement of incoming links to your site and it’s pages, this is true, but it’s far more important to note that it is a weighted factor. A small site, heavily targeted on a niche phrase and supported by varying, relevant, original content can, very easily, dominate a SERP with a PR of 1; while a PR 4-5 site may not be able to achieve first, or second, page results on a moderately competitive term.
I like to track page rank for one reason and one reason alone: When PR shifts, it’s usually due to overall Google shifts, not just my site.
If PR jumps or shifts (on any site, not just your own), one could expect the SERPs to follow. But there’s no certainty in saying that a PR shift will affect your traffic, as noted by David Naylor’s personal experience.