I've always valued them at exactly 0.

Like someone before noted, if any carrier goes kaput, so could your miles, there is no FDIC for this currency.

I think of them more as potential vs. kinetic energy, where like a gallon of gasoline I could get less than 10 mpg in a gas-guzzling SUV, or I could get greater than 50 in a gas-sipping hybrid.

The miles, like the gas, have a wide range of uses, "kinetic," when issued as tickets, hotel nights, even magazines, but just remain "potential" while they sit in my accounts.