If a guy was of the gambling persuasion and had access to load boards and a lounge they could probably treat a day during the holiday crush like a trip to the horse track. They'll know which flights might sell out, buy a ticket, wait at the gate until the airline starts asking people to sell their seat, accept the offer of a refund of the ticket price and additional vouchers or money and walk away with a tidy profit and then rinse and repeat for an entire day. The vouchers have value, the airline has to keep them on their books for tax reasons and they'd rather payout than have someone holding onto them for years, for the same reason the company you work for will want their employees to use their PTO and not roll it it over every year, it becomes a liability to have all of that money/debt sitting out there not being used.