You know there's more to it than just empty seats, right? If they are hauling cargo, that always gets priority, especially over non revs.

I've not non reved on other airlines yet (I was going to go on Delta but they told me at the ticket counter the flight I wanted was already over by four and it had checked in full so I opted for old faithful) but I never 'push' the gate agents. Unlike me, they can't just hit a mute button when it comes to snotty pax, so I always give what I wish for.

So far, I've never 'missed' a flight *knock on wood* and have even scored a couple complimentary upgrades.