I think the airlines go to great lengths not to involuntarily bump someone, since this usually creates a VERY unhappy customer, which any airline can ill afford. With the vouchers for voluntary bumpees, they allow everyone who really needs to take that flight to fly it, and in addition, make someone else happy at the prospect of a future free trip. Also, if you have a voucher for one or two people, you might pay for another ticket or two on that future trip and take the whole family. As an additional thought, there is certainly some percentage of those vouchers that are never redeemed (lost, forgotten in a desk drawer).