Its certainly quite rare for someone's travel plans (or a ticket they booked) to have U class anyway.
Uh, really? There's an
entire thread in this forum that would seem to contradict you.
FWIW, I've gotten
waitlist upgrades on LAX-CUN, CUN-LAX, ORD-SEA, and used a cert to upgrade SEa-MCO (that I had to take back in order to make an earlier flight). And there's plenty of U on the Hawaii routes, as well as DCA-LAX, and the Bay Area/Greater LA/Vegas routes generally have it as well.