Actually, that is a Boeing approved SOP for normal operation. Considerations for non normal use are in the relevant checklists. And it is there specificly to provide low speed protection on instrument approaches during manual flight, the throttle is still used manually. Only if the speed drops below Vref you will get autothrust engagement to make sure the speed does not drop further. Once the speed is safely above Vref it will automaticly switch back to ARM.
It can be confusing if you are not aware of that feature and if you are not trained to use it. If you are trained to used it however it is just an additional safety net.
You are right however that in many non-normal situations auto thrust usage should be avoided and that is mentioned in the relevant non normal checklists, usually the first or second step is: Autothrottle ... disengage.