Sure you get to have gravity assist you when you stand, but it's not free energy. You have to lift your body at the top of each pedal stroke before you can recover the energy via lowering it again. I agree with Danno - as a bent rider I can't stand, and what I notice is that I can't lift my hips away from the pedals, which limits me in how much of the pedal stroke I can have my knees in a less-stressful (straighter) angle.