Change either pitch or power, and the aircraft takes time before it settles down again into a steady state. Power primarily controls Rate of Climb / Rate of descent and Pitch primarily controls speed. Inclimb and cruise this is how we fly all aircraft.
On approach, in a light aircraft these changes settle down pretty quickly, and thus the pilot can still fly the aircraft as above. In an airliner, the time taken for the aircraft to settle to a steady state in too long for the approach phase, so it is better to think on the thrust controlling the speed, and the pitch the ROD.