But if you think about it, if the driver's motor isn't 100% perfectly balanced, then it'll get distroyed, because of the Earth's gravitation (if you mount if vertically).
You see, if it's mounted horizontally, then if the drive isn't balanced correctly, then there would be a sligh movement from left to right, but since the force to the right and to the left is the same (nothing else influences it), then it's fine.
BUT, if you mount it vertically, the unbalanched part till have a double-forder acting on it: centrifugal (due to unbalncement) and gravitation. This force will be greater on the bottom than on the top (because centrifugal force will add with gravitation on the bottom, but will substract from it on the top). So, the drive will have a greater deal of vibration, probably resulting in a much faster destruction.
That's how I see it