This is where the U.S. airlines win hands down... In my previous days travel on American or United if I arrived at the airport early or made it to the connecting gate I was put on standby for the earlier flight. I think the official rules was a small fee $50-70 applied if you didn't have frequent flyer status but I was never once asked to pay it.
It just makes good sense to allow passangers not necessarily to reserve but to standby for an earlier flight. If the 12:00 flight is leaving with an empty seat and I'm at the airport ready to travel with a ticket for the 14:00.
That gives them both an additional 2 hours to sell a seat that would have gone empty and additional flexibility to operate their flights. If the later flight is cancelled or delayed. That's one less passanger to find a seat, re-route or accomadate in a hotel.
I entirely agree with the above sentiments. If a passenger is at the gate and ready to fly, putting him/her in a seat that
would otherwise go empty just means that there is that much more time to resell the seat to a passenger with last minute plans, or who must travel on that particular flight. Charging a nominal fee is appropriate in some circumstances (i.e., passenger request, such as the traveller who shows up in time for the earlier flight), but does not make sense in the case of IRROPS, potential IRROPS or potential overbooking, where the passenger gets the airline out of a potential problem (even if initiated by a savvy passenger 'volunteering' for an earlier flight).