Individual magneto switches present a different situation to a rotary, combined switch. In an aircraft fitted with individual mag switches, I would certainly agree that a dead cut check is unnecessary.
Not necessarily true. If you take a Chipmunk as a case with individual switches although you have tested left and right it's very difficult to detect a RPM drop so all you've done is proved that both mags are working. You've haven't proved whether one mag is permanently live or not. I know I'm a pedantic old fool - sorry. But this very situation occured to my knowledge recently and the only way to eventually stop the engine was by turning the fuel off, which of course took some time. Certainly worried the pilot who thought he was never going to be able to stop the engine. No ICO you see!