Frequent flier miles sound nice but are notoriously difficult to redeem and accumulate simply by flying. The Washington Post reports that FF miles really serve as a vehicle for airlines to sell you other things.
Frequent Flier Miles Are Not Banks – Airlines do not pay interest on miles you hoard in your account. Worse, you could fall victim to frequent flier award inflation — there is no guarantee that the airlines won’t increase the price of awards while you’re saving up.
Since there are no government regulations of FF miles the airlines can do what they want with them and not notify you in the process. All of the major airlines FF miles now expire after a set time period so make sure you read the fine print. Don’t hold out for free flights, go for upgrades as soon as you’re eligible.