I see a similarity of this issue to the hotly debated (at least on the Southwest forum) issue of whether a customer should be able to standby for an earlier flight for free. On Southwest, and increasingly on other airlines, you have to pay a hefty up-charge to take an earlier flight if you are on a discounted fare.

Advocates of free standby correctly point out that the airline incurs virtually no cost for letting you standby on the earlier flight. Furthermore if successful you free a later seat for possible sale to another customer. All true.

However look at the problem using game theory and it changes. Once customers learn that they can standby for free, some of them will buy tickets on the cheap late evening flight rather than the expensive peak hour flight. That behavior change will definitely save those customers money and cost the airline money. Advocates of free standby know this and in fact it's why they advocate it, not realizing that the airlines would not be able to maintain their current pricing under a more liberal policy.

In summary, the right answer for a one-time, never to be repeated event is different than the right answer for a permanent policy decision.