The diversity mode,
AFAIK, is to support two aerials to improve visibility to other aircraft flying above/below you. The
enhanced Mode S installation in e.g. a TBM850 uses this.
You are right that the inputs can be disabled but if e.g. you bring in the GPS GS to get automatic AIR/GND switching, you have to enable that input to make that function work, and the GS is then radiated. The only "legal" way in the EU is to use a landing gear squat switch, or an airspeed pressure switch, to drive the GTX330, but these involve a lot more work than using the GPS GS, both actual and paperwork.
I am not up to date on this (all IFR pilots I know installed the GTX330 years ago) but Garmin were going to change the firmware to enable the GPS GS to be thus used but without being radiated.
In a typical GTX330 installation done outside Europe, you connect everything you have (the whole NMEA data stream, etc) to the GTX330, enable all the inputs, and it all gets radiated. In the USA, the more the merrier
