If you were told to maintain altitude and throttle setting how would you maintain airspeed? Only an idiot would expect you to jockey the throttle for every turn, especially if you are flying IFR. Making you maintain airspeed and altitude throughout the flight is an attempt to teach you to stay aware of what the plane is doing, but it needs to be tempered with the knowledge that a smooth flight for your passengers is also important. An instructor that was trained to 141 standards and is training to them as well, is going to train you to be smooth and precise. Airspeed can fluctuate with turns and turbulence and fighting it constantly is a surefire way to get your passengers sick. A 30deg bank you will loose about 2-3 knots through a 180deg turn in a Cessna 152. It will recover it as soon as you are wings level, but you still loose a little. If you were to enter a 60deg bank in a Cessna 152 fully loaded and he expected you to maintain airspeed and altitude then you go full throttle and hope its not too high a pressure altitude. If you loose airspeed but maintain altitude and he's mad, then again, he's an idiot.