The only real advantage I can see in having an Aircraft Operator Designator / Callsign is when using multiple aircraft to operate a particular series of flights.
If using G-ABCD, should the aircraft go U/S then the flight plan must be cancelled and a new one filed (slot delays?).
If using ABC123 then should the chosen aircraft go U/S then only a change has to be filed giving the new registration to be used.
It also helps to identify the flight as belonging to an operator or as an aircraft that performs certain special trasks eg police or calibration flights.
Someone earlier stated that an organisation paired callsign with aircraft - what a waste of effort!
From a practical point of view having obtained the callsign then all flights by that operator should use that callsign. The numbers used should be co-ordinated so that thay do not conflict with others on the system at the same time eg don't want a BAW002 and an AFR002 and a ABC002 all turning up on the same frequency.
G-ABCD can be abreviated to G'CD. ABC123 must alsways be ABC123 in full - think about R/T loading!
Is this not getting like the whole "personal registration plate" craze in the UK? - pointless.
Regards,
DFC