Yeah, it becomes a brainer, though, when you ask “how do you deal with malicious transmitters”, since the answer to that tends to involve developing the systems to monitor the same variables externally without trusting the transmitted values, and once you do that, what is he transmitted data for?
Using transmitted data for less safety-critical things like route intent for efficient holisitc route optimization, with a penalty system for misrepresentation might be useful, though.
Maybe you could have a lower power transponder style technology in addition. This would just broadcast an ID after a fixed amount of time in response to a request. This could be compared against radar,visual, and broadcast information to actively verify data. And critically it could be independent of the rest of the cars systems. Obviously that is not a solution, but it does add another layer of redundancy
For example, we could have a device on cars that transmit basic information. Such as licence ID number, speed, location, pedal position etc. It would be ridiculously useful data for a whole host of different things. You could use it to bill parking, catch speeders, sequence traffic lights, and warn of hazards ahead. And maybe even help self driving cars make sense of the world.
From a technological point of view this is a no-brainer. But this is incompatible with the way that we think about cars and roads. Every car has to pretend to be isolated from every other car.