To maintain a flight path: pitch
To change or maintain speed in a constant flight path: power or thrust.
In every airplane. In Trevor Thom's IFR instrument flying books is explained like that, for light airplanes.
However, what would happen if an AP in a 737 had the A/THR coupled with the altimeter to maintain altitude and the pitch with the ASI to maintain airspeed? This egg and chicken thing can be very puzzling, indeed.
I think that, for a human brain at least, it is easier to divide the tasks conventionally, pitch for path, power for speed.
The control and performance instruments concept is quite clear and it has been stablished long time ago.