I was answering with the purposes of flight-taxis in mind. I doubt that they will fly in high altitude and deem it quite likely that they might want to pass uncontrolled airspace at some point. And if you're flying in uncontrolled airspace you just can't count on the presence of it. In fact, missing transponders are an occasional source of accidents in these spaces. There are lots of gliding enthusiasts that aren't equipped.

P.S.: Commercial planes (esp. big ones) should have hardly any contact points with flying taxis.