At least for me, it's psychological. If I'm on the FG in a headwind, I'll spend a couple of minutes feeling sorry for myself, cursing the wind, and wishing I had the derailleured bike. Then, since I don't have the option, I'll suck it up, get down in the drops, and grind away. It's faster and in the end it helps improve my fitness, which is one of the reasons I built up the FG in the first place.