A few thoughts... I hope they're salient.
I'm certainly not in favor of legislation for its own sake. The current laws are difficult enough for the officers to enforce, and everyone knows it. But ticketing people for breaking a law is only part of the reasoning behind new legislation.
Many, many people who are currently using cell phones in moving cars will cease to do so to avoid a ticket, or just because they don't want to break the law. Just as many people who never wore seatbealts, do so now that it is illegal not to. Will it stop it altogether? No, but it'll help; and every driver that quits, lessens the likelihood of accidents, and saves lives.
Inattentive Driving is against the law, but most of the time offenders are not prosecuted for Inattentive Driving but for individual infractions: Running a stop sign, exceeding a safe speed, etc. If these laws are passed, I can guarantee that dash mounted cameras, and pole cameras will be used to prosecute offenders. So that a driver running a stop light in full view of a LEO or traffic camera, will not only receive a ticket for the stop light/sign, but for talking or texting while they do it. Likely receiving a predetermined stiffer penalty.
Hands free vs talking to a passenger -- A couple of things to think about:
When a person is engaged in a phone conversation, they are often at the mercy of a topic that requires a great deal of attention. They're discussing problems at work, technical issues, money troubles, personal conflict, or being issued commands that they must concentrate on and remember. While the same may be true of a conversation with a passenger, the major difference is that the passenger is actually in the car as well, and has their own personal safety to worry about, and will often cease talking when the situation around them warrants it. Not to mention that conversations with a passenger, may be focused on what both parties are observing outside the car as it moves. With a phone call, the person on the other end of the line is largely clueless about what the driver is dealing with, and may choose a particularly inappropriate moment to demand the full attention of the driver.
Personally, I feel that what would be most effective is not new legislation but to use technology (which is at least partly responsible for the problem) to help with the solution. How hard would it be to build a circuit into a cell-phone that will not allow it to be used in a moving car, with a huge penalty for anyone found defeating the device?