This is another case of a one-size-fits-all recommendation. If I am driving on rural freeway in a low traffic situation, I am much more likely to become drowsy from boredom. A phone conversation helps greatly in having just enough stimulation to keep me alert. While I believe the recommendation goes too far, I do support the absolute ban on texting--doing that while driving is crazy. And perhaps there is adequate evidence to prohibit using your hands to talk on a device, but banning the use of hands-free devices to talk makes no sense. That is almost the same banning conversation with your passengers. While they're on the subject of distractions, how about a study of the overly complicated electronic controls in new cars. I definitely cannot operate much of what is in my 2011 VW safely while driving.
Just like the illegal to text law, this would be hard to witness and prosecute. A cop looking into a car...how does he/she know the person is talking on the phone...or singing...or talking to an imaginary friend? Nobody knows. The ONLY way this is going to be prosecuted, if passed, is if there is a BAD accident resulting in death. Proving one was on the phone during a crash (or texting or loading web pages, etc), phone records have to be subpoenaed. That takes way to much time for a prosecuting office to even want to deal with, unless of course the end result was a death investigation.