Even though the majority of ground radars are Mode S, not all ATC systems use this to display the callsign on the controller's display.
The 4 digit SSR code allocated is paired with the flight plan database and THIS is used in some places to provide the required information on the radar display.
Therefore, both are still needed for now.
When I retired from Prestwick Centre 18 months ago, the sectors below FL255 used the SSR Code and database, the high level sectors used the Mode S on their newer system.
The low level sectors were able, using a mouse click, to look at the Mode S information, which was quite useful when a "7000" popped up in the FIR when we were providing services outside CAS.
A lot of them on the West Coast of Scotland were regulars who were listening out and we just called them and asked their intentions, which meant we could agree separation if necessary.
PS. It is not unknown for the pilot to input the callsign incorrectly, such as BA1471 instead on BAW1471 !
Last edited by The Fat Controller; 22nd Feb 2018 at 12:24.