I don't think anyone is saying to IDB a confirmed passenger. Yes, flights do get full but a flight that is booked full 4, 14 or 40 hours from departure may not end up being full when it departs or at least not full with the same people. Mis-connections, no-shows, voluntary changes all happen. That is why even if an airline can only confirm a passenger on a flight in, say, 48 hours, they should allow them to standby for every flight between now and their new confirmed reservation because there is a very good chance they will get out earlier that way.