Autothrottle during Manual Flight
One of the main reasons for disengaging the autothrottle when flying manually is the workload associated with having to re-trim pitch each time that the thrust setting is changed. This is the case on an airplane without FBW pitch control augmentation that has under-slung engines.
On the B777 and B787 the FBW augmentation provided in the pitch axis during manual control (C*U) does such a good job of adjusting the elevator to account for thrust pitching moment changes this is no longer a problem. The autothrottle helps greatly with maintaining speed so that benefit is preserved by keeping it engaged.