This is very true. You simply can't maintain high power, on variable terrain, without bindings. If your rides are billiard table flat, or big descents on to long flat bits, then sure, flats are fine. Your power will be cack, but your average speeds will flatter you. You need clipless to get a high, consistent power output, and to get a nice quick Cadence acceleration, and maintain that Cadence. The most efficient way of producing big power, being Cadence biasing your pedalling, owing to the fact that it's easier and quicker, to ramp your Cadence up, and maintain it, than it is to ramp the Torque up.