I think that overbooking due to no-shows is easy. If you bought the ticket and never made it to the airport you eat the ticket. The issue comes when the no-shows are due to missed connections because of airline or weather delays that are not the PAX fault. There will always be a certain number of these, and the airlines can't just void the tickets in those cases. A no-show will result along with the need to accommodate the customer on a later flight. I believe this was the original justification for over-booking.