I hear what you are saying but to play the Devils advocate for a second, is it not better to have the PinC sitting in the jump seat with a clear head ? The PinC is often not the most skilled stick and rudder pilot on the flight deck so is there possibly a safety benefit to having them sitting and observing and relaying the odd instruction when necessary?
However PIC is ultimately responsible. You can't be ultimately responsible sitting in the jumpseat. You could quite conceivably sit and watch two inexperience guys kill you. Air France 447 would be one example.
Unlike boating flying can unravel in seconds and the last thing you want is the guy who is in charge not in a position to do anything about it.