As the person buying an empty seat, I would sure hate to be providing extra space to another person (on the other side) for free!

Given the demand for this, it is surprising that the airlines flying a 3-3-3 configuration, such as the 777, don't offer a guaranteed empty seat for 150 percent of the regular fare. I would think they would sell plenty of these tickets, and it would provide a real competitive advantage. And far better to be paid for not flying someone than for flying someone.