Originally Posted by carleton

True, but the downside of wheel-based systems is that they are very far down on the kinetic chain. Power meters are supposed to measure what the body is doing, so ideally you'd want the meter close to the body to get the best readings. Without actually being on the body, the closest we can hope for is a shoe power meter, next best would be cleat, then pedal platform, then pedal spindle (LOOK, PowerTap), then crank arm, then chainring spider (SRM), chainring, chain, cog, rear hub (PowerTap). Some power is lost as friction at every step listed.

BUT, on the other hand, the rear hub is very close to where the power is applied to the ground. So, it could be argued that it's in a great position to measure what really matters: How much power was produced that made it through the bike and into the tire that propelled the bike.