Hmm, I dunno... it relieves some of the financial pressure of having empty seats, by extracting some value from them. Therefore there will be less incentive for the airline to maximize seat utilization (through overbooking, for example, which saves money but costs reputation for the airline every time someone gets booted). Therefore we can expect seat utilization to go down.
That's true. That's a pretty complex effect, so you'd need some decent data to estimate its magnitude. But you're right it should be above zero, unless there's some other effect to suppress it.