Yep. By Mileage Plus not giving unlimited Domestic upgrades to it's highest elite level flyers, Premier level members probably enjoy a fairly good upgrade success rate. Or at least a better upgrade chance than they might experience at some of the other airlines that do give unlimited upgrades to their top dogs.

As a result, you often see 1Ks sitting in the back when Premiers are being moved up front, simply because 1K's don't have an unlimited supply of Domestic upgrade media.