I can't find a Boeing reference, but I would guess it is because LVL CHG will not command a level acceleration segment but up to a 500fpm climb at least (watching acceleration two engines when cleaning up &c.)
Denti - how does your company permanent dual channel policy deal with disconnections below 400ft on landing? You are then manually flying an aircraft that's bloody miles out of trim which is hardly conducive to a stable approach.
I have only flown asymmetric for real once (in a bizjet due to an ECU failure) and I was far more comfortable flying manually once the QRH nonsense was complete due to being more in the loop and the nature of the failure. An autopilot without a rudder channel makes for a curious mix of automatic flight but manual handling of yaw and rudder trim and my view is when thrust and speed changes are involved it is wiser to fly manually than risk the autopilot kick out with a large handful of out of trim forces.