The only time I've ever been asked to show a card with a *G logo on it was a few weeks back when I was supposed to fly MCI-DCA-YYZ on US, and got last-minute rebooked onto AC MCI-YYZ due to delay/misconnect.

And that was only because US had literally just told the AC agent that they'd rebooked me; she didn't yet have all the info when I showed up to the counter with my bag, so in the interest of getting stuff processed quickly, I just pulled out my card.

(of course, the mileage for that still hasn't posted, but at least I actually got where I was going -- and three hours earlier than expected)