Carb ice can build very quickly, you won't even have time to sport it on the gauge. Because of the reduced airflow, ice can build up either on the butterfly and in the venturi. The temperature probe is located just behind the venturi so doesn't measure the butterfly and the most critical part of the carburator.
When you get it your engine quits and in a helicopter you can't start it again. In a fixed wing you've got better changes because the prop is connected directly to the crankshaft and keeps the engine turning.
The 12 C has a gauge as well, you use the gauge for inflight conditions. Almost every aircraft has a carb heat gauge.