Since Boeing recommends and large majority of airlines are not deselecting SPD on approach, I guess there must be something to it...
Regards your points ref use of A/T in ARM mode could I ask the question another way:
- why have Boeing included the ability for the PF to de-select the Autothrottle into ARM mode if this function is (apparently) never to be used?
Surely the A/T automatically going into ARM mode as you describe could be wired as a function of the on/off switch and A/T computer and other associated system computer logics etc?
Boeing has included a lot of features on the 737 that you shouldn't use as per their recommendation: A/T with manual flight (except during takeoff, climb or go-around), G/S capture before VOR/LOC (before respective SB), etc.
Aviation as all other industries learns its mistakes as times goes by and sometimes manufacturer's brilliant ideas during aicraft design turn out not to be so brilliant, so some capabilities of the aircraft are not recommended to be utilised in order to prevent some nasty situation to develop.