The psychological evidence is quite clear: using a cell phone while you're driving is distracting. Conversing with someone on the phone imposes a cognitive strain that makes it harder for the brain to concentrate on other tasks. Hands-free systems keep drivers eyes on the road, but they don't do much to reduce their level of distraction. No matter how you cut it, the case for banning drivers from using mobile phones is a strong one.
What's less clear is whether or not these bans actually reduce road accidents.