Although driving by it's self is inherently risky, it can be done safely with the proper precautions. Car accidents can be reduced to mechanical failure.malfunction and unforeseen medical emergency if everyone just paid attention and respected the drivers they share the road with. Driving while talking on your phone, updating your social networking, or sending/reading texts (hands-free or otherwise) is a distraction and by definition cannot be done safely.
There is also a difference between hands-free and having a passenger in the car (even a blind passenger). At any point in a conversation with a passenger, you can tell them to shut the fuck up and let you drive, at which point the passenger can take care of themselves until further notice. They also have some form of sensory input that allows them to interpret the situation at hand and assist you in either shutting up or providing relevant information to the crisis at hand. A hand-free device, Siri powered or otherwise, cannot interpret your current situation. If your trying to navigate through congested traffic and need to concentrate, Siri might still be reading you the list of texts you asked for, not realizing it was 20 texts long. Siri might not understand you when you ask it to stop, and it has no sensory information that it can accurately and efficiently relay to you.