This tip was published in 1995 by Robert Prechter in his book, "At The Crest of The Tidal Wave"....page .103
"The airlines issue credits to reward people who fly, order flowers, take a language course, stay in certain hotels, rent cars, call long distance, or even invest in a mutual fund. This frezied purchase of customer goodwill has been paid for with future obligation, so it is simply debt, a huge debt. Already, nonpaying flyers account for 7% of the seats filled on the average flight by major carriers. When the depression arrives, that percentage will soar. The industry's debt to its best customers is now huge, and its ability to pay it is negligible. The problem is just beginning to show up in airline's policies of further restricting the time and seats available for certificate redemption. This is only the hint of the default that is coming. Any renewed contraction in the economy will devestate the transportation industry. If you have accumulated any frequent flyer miles, you should use them up as fast as you can."