I have booked 4 free trips on UA this year while buying none (so far). Last year 1 booked 4 paid trips on UA and 4 free trips. (and 3 free trips on AA (and 2 on B6 (my sons points paid for one of them) I am not an airline expert but seems that flying for free cannot be good for the airline (I get most of the points via the CC on spending I would have done anyway). At least with B6, when I am stuck paying for the high december/february fares I get more points out of it (as compared to UA). Delta is adding a revenue requirement for Medallion status and the other airlines may try to do it indirectly (via minimum flight requirements).