I'm not a big fan of too many intrusive laws, but, to me, this one's a good one. The information is out there: Cell phone use (including texting) is a factor in a lot of accidents (and serious ones). People shouldn't need a law to be smart enough to risk harming someone else (or themselves, but they aren't the only ones who may die if they choose to believe they're better able than "everybody else" to use the phone without being distracting).
People "shouldn't" need a law; but too many are arrogant, naive, stupid, immature, or whatever else they are; so too many apparently have a hard time just holding off on using the phone until they're parked.
The laws still won't stop everyone, but at least they'll discourage some people from using the phone when driving, and at least there will be just that little bit more awareness that driving and using the phone isn't as harmless as so many people think it is.
It's not that difficult to just pull into a parking lot if a phone call or text message is "such an emergency". When cell phones started to become as widespread as they are, it would have been nice if everyone could have been "grown-ups" in their use of them - but not everyone is a "grown-up", so now we have these laws that are aimed at trying to reduce the loss of lives that has been increased by cell phone use/texting in cars.