A further advantage to the airline of the voucher/free ticket is that not all of them get redeemed. A small number are forgotten about or the passenger ends up not wanting to travel again, a larger number find that when they come to try to use the voucher they are told "no space available". Airline reservation systems long moved ago away from just seat availability to determining who you are before allowing a reservation. There are many accounts by those who find vouchers or miles to be completely unusable, even though flights are going out with empty seats.

One day the US DOT will wake up to this one and require that VDB/IDB vouchers are able to be used on any service with available seating at time of reservation, not just sticking them against unusable fare classes.