I get the opposition to being required to carry a transponder, just to ride a bike. My intuition is that so many people would balk that it will never happen, and if it did it would be unenforceable.
But turn it around, if new cars including manually driven were required to detect them, and present some kind of warning or cue when a signal was in or close to the vehicle's path, I'd gladly purchase one and carry it around. Pedestrians and cyclists who didn't, would be no worse off than before. It will never become the sole or primary way for vehicles to avoid hitting people (just a little common sense here) and will always be at best an additional safety measure. In that case I don't see any problem with the technology, and would be glad to see it implemented.