There are five-odd roles on the flight deck:
- Pilot handling controls
- Pilot preparing/briefing approach charts/nav radios etc
- Person manipulating the navigation systems (the old Navigator's job)
- Person running the mechanical system switches (the old flight engineer's job)
- Person working the radios (the old radio officer's job)
It is the Captain's job to assign those roles according to the requirements of the time. The jobs are not of equal workload, and increase and decrease in workload as time and situation demands.
You can always tell a Captain failing the task assignment portion of "CRM" in the sim when you see one person working their ring off, while the other sits staring out of the window. (This normally happens as the abnormal checklist is completed (that task suddenly becomes smaller) and the Captain fails to re-assign the task load, so the other person is flying AND programming the box AND working the radio AND trying to brief ...)
Now I have no problem with pre defining some role assignments, it helps to have an idea - but
prescribing roles is a bad idea - all situations are different. Sometimes you are flying with a well experienced good flyer but new to type (so not so solid on the mechanicals), sometimes you are flying with a cadet just off rating, so not so hot on handling, but brand new solid mechanical knowledge etc etc.