if it were to be optional like a button on a dash and was to slow the car down if you were heading to something in front of you too fast to possibly stop in time it would be good but i dont see the point in it other wise if your taking your foot off the accelerater chances are your about to brake anyway :S
Braking to a complete stop? what if someone pulls into your lane on the motorway in front of you or similar, then you lift off the gas to let them have some room?
The PC takes over and stops the car on the motorway!
The alternative is to keep your foot down and smash into the other car!