Extract from B737 FCTM re manual flying and A/T:
"To simplify thrust setting procedures, autothrottle use is recommended during take off and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged...The autothrottle arm mode is not normally recommended because its function can be confusing."
While that may be true the autothrottle arm mode provides alpha floor protection. In my experience it isn't confusing at all although different versions of the same aircraft (all -300) behave very much differently when approaching a selected altitude in manual flight with AT in arm (descending), according to our techies it's down to different MCP and FCC versions.
Anyway, it is SOP to leave the AT in arm mode, the rest as you quote it above.