"If one occurence of a linked URL is "not trusted" why would a second one be trusted, anyway?"
Because such a practice would be idiotic. Such illogic means you could ruin the PR distribution of any blog or forum site you wanted simply by adding the site's main page link (or their entire navigation) to the blog comments on any page.
"It makes quite a bit of sense."
It make no sense at all. It would be pretty hard for Google to do anything more bizarrely destructive.