I'm of the opinion that, in and of itself, a link is a link, PR is PR, and relevancy is something else.
In other words, a link is an entity in its own right and carries a base value.
PR and relevancy may add value to a link. But the lack of PR or relevancy won't reduce a links base value.
The impact of PR on a link varies depending on how many outbound links appear on the page. Of course it also depends on weather the link passes PR at all.
Relevancy is harder to pin down.
If a widget info site links to a widget dealer site there is not much question about relevancy.
But if a site about expensive antique widgets links to a site about travel it gets a little less obvious. Unless you realize that antique widget enthusiasts often travel great distances to antique widget events.
At the high end of the antique widget world you might well find folks with a big interest in the finacial markets, so a link to such a site might actually be relevant, while on the face of things it might not be so obvious.
You can stretch relevancy out almost to absurdity, and quite possibly beyond.
But how do you measure it?
That's why I think a link is a link and has its own base value that is not affected by PR, relevancy, being from an authotity site, and who knows what other factors.
Of course, I could be wrong.