Must be a bit difficult making entries into the FMC....
As a Captain in the left hand seat it would indeed be far worse to have your right arm missing rather than the left which would be a bigger problem.
Flying as an FO the opposite would be true.
Working up to flying as a Captain and starting in the right seat I would imagine loading radio frequencies FMS and even sliding the mike button( sometimes difficult and sensitive with normal limbs) and all the other challenges required of an FO would be much harder.
I am not qualified to know how sensitive these false arms are to making such fine changes as not only do loading frequencies require delicate pushing movements but also twisting movements.
you have a valid point which maybe someone who does know can answer