If autopilot OFF, A/T ON is so complicated, how come that's how USN pilots do a lot of traps that way? It's not a problem; except I would submit on an Airbus where the throttles don't move.
They do it because (1) the airplanes are designed for it; and (2) they practice it a lot.
Flying an autothrottle approach to the carrier is a LOT different than a manual approach. It requires a finesse with the stick in pitch that is different than a manual approach. While the newest airplanes (F-18x) may have the differences ironed out in FBW software, the older (A-6, A-7) airplanes required distinctly different techniques.