It's definitely useful--though IME it takes a kiosk to trigger the offer. The online site doesn't seem to do so in those circumstances.

As a side note, I've been waitlisted for upgrade with RPU several times when R has gone positive once within 24 hours, and didn't clear; I had to call an agent. I know there are apologists who say, "Just wait, and it will happen," but given the way R > 0 seems to trigger upsell offers, that's not a chance I'm willing to take. And if in so doing I bypass others waitlisted with instruments, so be it; UA should fix their crappy upgrade system!