What would that be used for? TCAS in flight?
As a general aviation safety enhancement, maybe. But it's not going to be useful for training purposes.
Not everyone has ADS-B out yet, so if you're looking for some sort of TCAS functionality I think it would be better to have a device which detects transponder returns. Not a bad idea at all in a congested area.
Airline pilots don't use ADS-B directly. It interfaces with other systems behind the scene.