Pitch for speed and thrust for altitude can in certain situations delay the re-stabilization of the aircraft due to an incorrect initial action. Take, for instance, an aircraft that is high and fast on approach. If pitch for speed is used, the aircraft will be pitched up and thrust will be reduced. As the aircraft slows down, it becomes more efficient, and the descent rate will be (much) less than in the other case, delaying the stabilization of the approach. If, on the other hand, the aircraft is pitched down towards the glidepath and the thrust is reduced, the speed will remain higher (but below the applicable placard speed) and as a result the drag will be higher. The descent rate will be higher and a stable approach will be achieved in the shortest possible time.
Pitch for speed etc. is a simplification that may work for small aircraft (and delta wings, but they are a special case), but if you think about controlling the aircraft as a form of energy management (the amount and balance of kinetic and potential energy) one thing that becomes clear is that you will initially always need to pitch towards your desired flightpath, and then adjust thrust for speed. This assumes that you are in control of the aircraft, so not in a stall.