Sheppey, both reasons you heard for this practice do sound like a load of old bollocks. Back in the day when this became SOP it is unlikely that psychology had found its way into the airline world.
More likely It originated with really heavy unpowered flight controls in large transport aircraft. The flight engineer would set power, then the captain would hold the top of the levers in preparation for a reject if necessary. At V1 the captain would need both hands to haul the heavy beast into the air, but the engineer would still have the throttles. Then the engineer went the way of the dodo, and the surviving dinosaurs retained the practice.
But if it’s SOP, woe betide the pilot who challenges it, or worse, disregards it. Always remember that he who pays the piper calls the tune.