I've heard this argument before, but it doesn't make any sense. It seems that you would lose more energy this way than with a freewheel, because of the energy loss to the complete drivetrain system as the energy is transferred to the chain and then to the crank arms, verse the rearwheel on a freewheel which would only lose energy to the freewheel itself. I am not going to argue that climbing isnt easier on a fixed gear, but I think this is because you don't lose any energy in your pedal strokes in the gap between your cadence and the momentum of the rearwheel (if you could pace yourself properly going uphill it shouldn't make a difference whether you're geared or fixed). This, however, has nothing to do with the bike pushing itself.