PageRank is not completely irrelevant. It is still an important indicator of penalties that have been applied to individual pages. This doesn't apply to new web pages, but does apply to web pages that are more than a few months old.
For example, for almost three years Google has been on a vendetta to penalize link sellers. Almost all directories have a gray-bar PageRank (no value) for all of the inner pages. When there isn't any PageRank applied to a page, there isn't any link value to pass to another web site. The PageRank of a directory's home page is irrelevant. That only PageRank value that gets passed it the one on the page where a site's link appears.
Almost every "link page" in a web site also displays a gray-bar PageRank. Link pages are pages with links to other sites, but have no real content on the page.
Although Google doesn't officially call it a penalty, duplicate content also results in a gray-bar PageRank. Autoblogs, sites set up using Yahoo answers content, and other duplicate content almost all show a gray-bar PageRank for all of the pages with duplicate content.
PageRank can also be used as an indicator that link juice is flowing to inner pages in a site. If the page is buried too deep in a site and has no external links pointing to it, it will have a gray-bar or zero value for the PageRank. Ideally, every page in a site that is more than about 4 months old should have a PageRank value, even if the value is zero. Gray-bar PageRank indicates no value, which is worse than zero.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin