Why does the acceleration matter if we already regulate velocity?
If I see a car stationary at a junction, I can assume that 0.5 seconds from now, it will either still be stationary, be a little further forward or a little further back.
The range of possible positions is significantly reduced with an acceleration limit.
If some cars stop better than others, then if everyone slams the brakes on, the poor souls in cars that don't stop as well will collide.
Keeping a safe distance between cars mitigates this though.