Boeing have already given their advice via the FCTM. You must assume therefore that their test pilots have researched and test flown the procedure very carefully. If Boeing had to explain in full detail all the reasoning behind all of their recommended procedures, the FCTM would be double the thickness.
For ages the Boeing 737-200 did not have autothrottle nor alpha floor protection (or whatever you wish to call it). Flying the 737 was not labelled dangerous because of this.
Why on earth cannot people accept that Boeing do know what they are on about when they explain that operating the autothrottle in ARM while flying manually is not recommended unless during take off and climb. During all other phases of flight autothrottle use is recommended only when the autopilot is engaged. Accept this very sound advice. Live with it. Stop reinventing the wheel etc etc.