This is really interesting from a game theory perspective.
What's really going to matter is how it's executed. If someone bids $100 to keep a seat empty and the flight is empty, do they still get charged the $100? Or is it only when there's a limited number of empty seats and more people that will pay to have an empty seat next to them.
One interesting way would be that each person gets charged the lowest amount of everybody who has an empty seat next to them. This obviously limits what the airline makes, but keeps things generally fair.
Another way is when people go to reserve seats, if someone is willing to pay $x to keep an adjacent seat empty, the price of selecting that seat goes up by $x. This guarantees the airline gets paid at least $x as soon as someone bids that. It also makes it more important to book seats early, but will of course, get much more complicated to execute.
One of the things I do when booking flights for two, is choose an aisle seat and a window seat. Odds are if the flight is not full, a solo traveler will not book the middle seat, and we end up having all three seats for us. 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.