Are these something from nothing? I seem to think so. This is why I found this advice odd. CN Traveler claims that 1 mile is worth about 1.2 cents (although estimates from SmartTraveler are higher). But then it says:
12. Don't squander your miles. Save them for special trips or expensive flights—don't waste them on tickets you can buy for a few hundred dollars.
13. Save your upgrades for flights on which premium class is truly premium. Use them on long-haul routes, say, or to upgrade to first class on a three-class airplane.
My thought was just to use the miles for whatever trip I'd like to take (long or short) before they expire. Irrational?