From the 737 FCTM: "To simplify thrust setting proceduers, AT use is recommended during T/O and CLB in either automatic or manual flight. During all other phases of flight AT use is recommended only when the autpilot is engaged."
I guess this is why we disconnect AT but I have to admit never to have flown with AT engaged during manual flight. I recall something about PIT due to AT engaged in manual flight? Anyway, I think the ARM technique on approach sounds useful during GA.