Some state that using hands-free is no different than talking to someone in the vehicle. I've read studies that show that is not the case for most people. When talking to a person in the vehicle your mind is in the "here and now". On the phone your mind is wandering to the other end of the conversation, especially if it is a heated conversation. I've had it happen to me years back when I was chewing a customer's ass that was trying to penalize my employer for a mistake THEY made. I was about 10 miles past my exit when I realized I had blown by it.
I'm sure many people can handle it, but my little 12 mile commute in the morning and evening usually produces 2 to 5 examples each day of those that can't!