Why bother with transponders when most new transmitter/receiver technologies already have unique MAC-address-like fields, precisely for the purpose of identification?
I guess its for the 'flew dangerously and then got away with it' case, where some followup/law enforcement would be required to nab the perps, and not so much for the 'lets find out who is flying this thing right now and where they are located' ..
For the situations where a drone is flying autonomously or out of range of a transmitter; You could still use ADS-B type transponders to track it and avoid it.
I guess its for the 'flew dangerously and then got away with it' case, where some followup/law enforcement would be required to nab the perps, and not so much for the 'lets find out who is flying this thing right now and where they are located' ..