I guess the main distinction will be the operation during normal and non-normal procedures. Remember during non-normal situations, both PF and PNF have very distintive duties. Generally, PF is concerned on getting the aircarft down safely while the PNF provides all the support.
Thus it would seem that Mr Boeing feels that during high stress situations the PF should not be distracted and the PNF takes on more duties.
Well thats my opinion anyways...