PR is just one way of getting links. It's often hard to discern whether or not specific links are actually helping or *how much* they are helping when we factor in the time delay from when the search engine picks up a link, to the time we eventually see a ranking increase.
This problem is compounded when we are using multiple link building methods at the same time, which most SEO's are usually (or should be) doing.
For me, I would never discount the value of a press release, which is a highly relevant piece of content, that I have published on a high ranking website with search engine friendly links (and syndicated across hundreds or thousands). I would be cautious of using obvious SEO links in them, but I would not consider that a worthless use of link building by any means.