The lower body, to me, is as important, if not more important, than the upper body. We use the Power Bat and the Hitter's Seat to teach lower body mechanics. I've also used the Perfect Stride to teach footwork. As a former golf professional I know how important and how difficult it is to teach lower body mechanics. That's why I like drills, tee work and soft toss to establish a good foundation.
Rotation occurs after the front foot has planted and the weight has stopped shifting forward.
Therefore, the head and eyes are in the best position to track the ball to contact. This will result in increased accuracy and consistency in the swing. The importance of this absolute rest in the fact that the less the head moves the greater the chance of sweet spot contact. Also, when the hips are rotated correctly, in the proper sequence and at the correct time greater bat velocity will result. This is difficult to do during live BP or in game situations.