Studies have been repeatedly conducted that show cell phone use while driving can be as dangerous as drinking under the influence. While other laws like "distracted driving" might arguably allow police to charge drivers, the lack of a specific law will result in the courts being tied up with unnecessary appeals. A professional driver might claim that they deserve to be let off because they're capable of driving on their cell phone. Another person might ask to be tested to see if they are really distracted when using a cell phone.
Banning cell phones while driving, as a rule, is done because the vast majority of people can't do so safely. And among those who think they can drive safely while talking, the majority of them believe so falsely. People often tend to be overconfident about what they are capable of doing. It's regularly seen with drinking and driving cases where people, including some reading this, believe they are "alright to drive while drunk because they're careful or good at it." These people are simply lucky they aren't caught or, even worse, being sentenced for killing someone.
My Dad is far more educated than average, and he has above average athletic ability. I've seen him do irresponsible things on the road because he was talking on a "hands free" cell phone. People like to pretend that cell phones being banned means we have to ban talking to passengers. This is not true, and there is little evidence to believe the situations are the same just because they both involve talking. The point is, though, that people are more than capable of making mistakes because of cell phone use.
Furthermore, cell phone use can influence traffic. One person will lose track of their speed while another one will decide to "drive slower to be safe" while inconveniencing parents trying to go home to their family. It's simply inconsiderate to risk the lives of others when, if you asked them, they'd probably want you to put the cell phone away. There is something...