Some airlines ARE operating this waiting list idea!!!
PIA, eventhough on their tickets have overbooking warnings, will put you on a waiting list if the aircraft is full. and if someone doesn't turn up then the waiting list Pax are isseued their boarding passes.
you can check seat availability and and if you are booking online, it actually tells you weather you have a confirmed seat or weather you are in waiting list.
A few years ago they denied my elder brother,with OK booking, boarding at LHE on Amsterdam bound flight only to accomadate someone with waiting ticket and friends in higher places...And as a result my father took them to a Pakistani court and received an appology plus a return ticket for ANY international journey plus court costs!!!
So my point is that waiting list type tickets will only work if airlines respect them and not for likes of PIA!!!!