GF,
I believe that Boeing, at least, has a concern about manually responding to autothrust induced pitch changes in the case of under wing mounted engines. This may be why the 717 FCOM allows autothrottle use when in manual flight; on the other hand, it may be an artifact from Douglas that has not changed.
Boeing is pretty clear about their preference for manual throttle during manual flight on the 767/757. That said, none of the operators that I have flown the 767/757 for actually adhere to that; typically, the autothrottle remains engaged until quite a bit closer to the runway than the autopilot. The concept that I was trained with was simply that if your throttle hand moves forward, then your flying hand moves forward, and vice versa.
The Boeing explanation of eliminating speed additives during autothrottle use is that the autothrottle system is very aggressive when increasing thrust in response to a speed decay, while it is rather lazy about reducing thrust in response to a speed increase. Therefore, it will operate conservatively in dynamic situations, resulting in an average speed somewhat on the high side of bug.