With regards to the question over whether it's impossible to rear end someone, I think the brochure and manual blurp say something like " within System limits".
Obviously if you are travelling at speed and someone pulls out in front of you, the laws of physics still apply, thus if the forward inertia of the car is too great to bring the car to a stop before the collision (given the braking and tyre limits), then yes, you will still hit them, but the system will have scrubbed off some speed so the collision potentially won't be a serious as if you hadn't braked at all.
Similarly, if you are driving in very wet conditions, icy or slippery conditions, or if the vehicle is aquaplaning whilst braking, then it physically won't be able to stop you, but should still lessen the impact.
Like 5-O said, if all cars had it, perhaps the chances would be much slimmer (as you wouldn't get rear ended and slammed into the car in front (that you have just missed).