At AA, agents are instructed to deny checkin to any individual that shows up with out a flight listing. In part because we want the work of creating a PNR handled in the back-office, not in the operation. Our agents CANNOT norec any more so you must have a PNR in the system before they can check you in. Further, I believe there are security issues involved which require a PNR.

As more and more airlines move to electronic ticketing for ZED type travel, a previously created flight listing will be mandatory to ensure the ticket is "tied" to the itinerary and available for check-in and lift. This means more and more airlines will be using web based technology to accept flight listings. I know several are working on it.

BA, like AF, have their own web listing tool which requires you enter you ticket number as your password. Several others use FlyZED, for now, but are likely moving to a different platform.