That said, no one lands manually with the autothrottle engaged
Well we do! We have no limitations on hand flying with the a/t engaged.
As to the first question I would suggest that if it isn't mandatory it is good airmanship.
Taking the OP's point about saving a landing by having both hands on the yoke, how about saving the landing with a go around? Can't do that without having a hand on the throttles. I know some of you might say that you can easily reach over and press the g/a button but try and find it on a dark, bumpy, !!!!ty night before you hit the ground after a late go around decision.