I think GrayAnderson was only thinking of mainstream (from legacy only down to conventional LCC) airlines. Such airlines can't afford to substitute last-minute sales at giveaway prices to fill the last seats, the way a ULCC (like RyanAir or Spirit) can, because the illusion of pricing power that filling unused seats with miles gives is broken if the seats are instead filled at $1 each or whatever. A ULCC doesn't care about pricing power, a legacy and even a traditional LCC like Southwest does. So everyone except the ULCCs need a "smoke and mirrors" way to fill up the last seats, without lowering prices, and no one has figured out another way to do that "smoke and mirrors" other than through an FFP of one sort or another.