Something about the PF using the PNF as his 'personal autopilot' is the way it was explained to me.
Witnessed it as a guest in the Flight Deck of a 747 classic and although it seemed strange it did work just fine (of course).
One of the oddest moments was when the PF handed back 'control' of the a/c to the PNF at the top of descent.
I do quite a bit of Duty travel with BA and the whole operation seems pretty damn good to me (I'm CX 744).