I find the privacy concerns here overrated. If you are afraid of being tracked, disable your transmitter, or spoof your vehicle ID, etc. Go ahead, who cares. Hopefully you'll also have the ability to buy a third-party device (or build your own) which lets you do stuff like this, instead of depending on the original manufacturer who probably won't. While the Fords and GMs won't like alternate suppliers, they're also not likely to provide retrofits, so an aftermarket might spring up to handle older vehicles... and user-preference scenarios. Does NAPA have lobbying power?
The real benefit seems to be with new robo-cars (driver-less), or commercial services like taxis & uber, big things like trucks, and utility functions like police, firetrucks, ambulances - which would benefit from broadcasting their INTENTS of direction, speed, maneuvers, maybe even destination, etc. Also smaller vehicles like bicycles and motorcycles which may want to broadcast just to be noticed, as their accidents often involve other drivers who just don't see them.
How to RECEIVE and process this incoming data is an interesting question. Maybe an augmented windscreen, showing small color-enhancements of what to expect, and warning indication of impending hazards? There's also automation potential in some form. I assume reception will be free and without required login, ie, the familiar broadcast model.
Security against hackers is certainly a good topic, but just part of the picture of bad data. What if a real registered driver intentionally give "fake indications", or merely makes a mistake in giving it, or simply changes her mind?
I recall a cop saying that turn signals are only suggestions; they can fine you for not using them (or doing so falsely), but you are still liable for accidents if you believe someone's mistaken signal and trust that instead of doing diligence in watching what the other driver really does. In other words, it's a mandatory but still advisory system. Contradictions and displeasure aside, at least we have a working model to build upon.