The "callsign" information attached to the radar return displayed on our screens in TC is not Mode-S derived. It is simply the flight-planned callsign converted from the squawk the aircraft has selected (relies on correct selection of squawk from the crew). To access the Mode-S I/D we need to manually select each radar return, which then displays the Mode-S info in a separate section of our screen. We are busy controllers, so to save manually selecting (wrestling with a roller-ball!) each aircraft we want to identify, we instead just use the more traditional method of "squawk ident".
I could also mention that most of the crusties probably don't even realise we can identify using mode-S, but I wont...