FWIW
At a flight testing symposium held by Boeing for my airline many years ago this question was asked of the experts. Their answer was that the FCTM is quite correct and they will demonstrate why they do not recommend the A/T in ARM mode in the simulator. I was not high enough up the food chain to get to spend some time in the sim with them, needless to say not long after that my airline changed to All On or All Off for the B737.
The latest FCTM still has the statement "Autothrottle use is recommended during takeoff and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged in CMD."