The argument that handflying causes the other pilot to be overloaded only stands when the pilots in question were trained with automatics only
I would not agree with that.
If you look at the average (good) multicrew SOPs and you will generally find that during manual flight the PF has hands on the controls and the PM;
tunes the radios, sets the mode selects, arms modes, does everything with the FMS, crosschecks the navigation, fills in the PLOG and handles the communication.
Hand flying is as easy as flying on autopilot especially in a long straight climb. In fact it is easier because the other guy is doing everything at that stage and one is simply making minor adjustments to a trimmed aircraft.
So it should be easy to see that when there are frequent frequency changes, route changes in the FMS, navaids to tune cleared levels to set, headings to set, speeds to set and so forth, the single pilot doing all that can get a bit overworked while his friend sits there leaning on the controls.
Anyone not think that in such a situation a more equal workload such as when the autopilot is engaged would be safer?
Finally, I again ask - does anyone remember the requirements for flying in RVSM airspace - I am directing this specifically at those that claim to regularly hand-fly the whole flight.