Alas, boeing is quite happy to change that advise quoted above if they are happy with additional training on the 737 that covers the AT arm mode. They even issue a different FCOM in that case that says about AT use:
To simplify thrust setting procedures, autothrottle use is recommended during takeoff and climb in either automatic or manual flight. During all other phases of flight, autothrottle use, other than in the ARM mode, is recommended (if serviceable) only when the autopilot is engaged.
The autothrottle ARM mode feature provides minimum speed protection in the event the airplane slows to minimum maneuvering speed.