I think it is unsafe, mainly because I'm frequently following people on the motorway who have poor lane choice, poor lane discipline or haven't paid attention to the exit they want until the last moment.
Apart from mainly being BMW drivers

most of the folks I see driving dangerously are on the phone.
I saw an A6 swerving all over the place the other day on the M1 because the bloke was using his phone and also tilting his whole body into the passenger seat using the dashboard tohide the fact that he had a phone in his ear.
It's insane. Get a car kit or a bluetooh HS.
I don't doubt that everyone is capable of driving one handed and that we do it all the time. However, there is something psychological about being on the phone and your concentration levels; just like most people can't most people give the phone thier upmost attention regardless of whther you are driving or not.
As for it only being as dangerous as talking to the person next to you, that's not true either. I can't be bothered to go into the psychology of communications, but it's easier to talk to someone you can see and you don't have to concentrate as much to make your point completely understood.
Talking on the phone does take more concentration.