Boeing 737 Classic (and 75/76?), has no FBW, so there is no compensation for the pitch/power couple caused by the underslung engines. Therefore when flying manually, Boeing recommend using manual thrust - so you know to compensate with elevator input and trim when adding or reducing thrust.
Airbus FBW however, does compensate for pitch/power couple, so it is entirely practical to retain A/THR when flying manually - it accelerates and decelerates neutrally. Airbus FBW also has a feature on approach called Groundspeed Mini(mum), which for example might suddenly increase the thrust a lot with a headwind gust, (to maintain a groundspeed rather than an airspeed). This is sometimes not very intuitive to follow in gusty conditions, so it can be easier to let the A/THR take care of it.
You can fly an Airbus completely manually though.
@FlightDetent, Surely when following a glideslope, (the OP’s question), pitch controls path, thrust controls speed?