Map = situational awareness.
Loc capture = Map update.
Classic (non-GPS) = possible map shift uptil Loc capture.
GPS a/c = should be very accurate all the time.
PFD Loc & G/S = raw data back up, but small scale.
SBY ADI ILS = Full scale deviation nof Loc + G/S.
Thus Map + SBY ADI has all the info you need in a neat package. Both pilots are looking at the same info. Was this not one of the debates about HUD's? LHS pilot flies HUD, RHS pilots monitors via raw data. They are not looking at the same info display.
The use of APP mode on HSI (ND) IMHO is only relevant if hand flying, and then only with no F.D. It is invaluable to lay the TK line over the ILS display. Piece of cake. Personally I also find this easier when flying a SE ILS even with F.D. It helps me 'lead' the FD and be proactive to prevent to many FD corrective commands.
The business of not knowing your deviation on the PFD/ADI outside 1 dot is a red herring. If you are flying manulaly with the F.D how have you allowed the a/c to be so far displaced?