What possible incentive would an airline have to do that, then?
> If someone does book the middle seat, we just trade aisle/window for the middle, so we're together, and he other person feels like he got an upgrade.
This infuriates me. My wife and I traveled cross country last month and due to a maintenance issue the previous night, our 6am flight was cancelled and we were pushed to 10am. What resulted was us trying to select seats when every row on the plane was either full or window-aisle. Had people flying together actually selected seats next to each other, it stands to reason some of those would have opened up and we may have been able to sit together.
Not a big deal for a 30-minute connecting flight, but not the best 5 hours to start a trip with.