I'm sure it happens. I see it all the time. While coach may be cramped, by stealing an upgrade, you make the process inefficient. You want to fly first class, then make a few mileage runs to become an elite member.
On the other hand, I am guilty of letting my wife sit in FC in my upgraded seat while I go back in coach. I don't think that is stealing, though, because the FC seat wass rightfully mine and I just traded with someone else. I have never just had one of us sit in an empty seat.
It is not only wrong (not a mortal sin, but still wrong) but makes the blood of those who witness it boil. This seriously contributes to the tension of travelling.
On a side note, what do people feel about the morality of moving to a better seat in the same price range at a sporting event if the seatholders leave midway through?
[This message has been edited by BoSoxFan45 (edited 08-28-2000).]