I'm not sure when that ended - US and DL had their "guaranteed seat" but the offer was for either another plane/crew or X (miles, free ticket, etc) and I doubt that either pulled out another plane for 1 or 2 people. Even Eastern, with the introduction of new jets to the Shuttle, realized that extra plane/crew even for 1 person would wreck havoc on revenue/expense ratios for the Shuttle. And that was when a new 727 cost 4 or 5 million so you can imagine what effect a $40 million plane sitting standby would have. It only really worked when the extra plane was already paid for and mostly depreciated, hence the older planes that Eastern used for the standbys - Constellations followed largely by Electras.

Jim