However it is a good practice during LVOs (T/O and landings ) to have the PNF use the ILS frequency on compass rose to detect any deviations and guide PF in case a sudden loss of visibility(fog patches)then the runway heading is used as reference along with the loc from the pnf to keep centerline
Let me see now. Aircraft hits really thick fog unexpectedly during take off/landing roll. PNF "Hey Cappie! My PFD shows you are deviating from the centreline - do somefink QUICK!
PF: "Speak up my man I can't hear you because of noise of reverse (landing)
PNF: "For Christ's sake turn left -aw bugger it I mean right:
PF; "Make up your mind man - speak slowly now..which way should I turn.."
Aircraft departs side of runway and kills a million rabbits.
Moral of the story? If you are concerned about loss of vis on runway then its the PF that should have the localiser up - not the PNF. Better still BOTH pilots have the localiser up?