What a modern ATC system (such as Eurocat/TAAAAAATS) displays are a different matter altogether --- both volumes of airspace and returns displayed are selectable. "RADAR" coverage and what a working controller sees are not necessarily the same thing.
Lead,
You seem to be implying that controllers do not always take advantage of all radar coverage.
Every sector displays ALL surveillance capability (radar, ADSC, ADSB) available for the airspace being worked at all times.