Its over kill for a car to have vented disks on the back wheels of a car. More than 80% of your braking come from the front wheels of the car just as in a motorcycle, where the rear disk is only half the diameter of the front. I believe that if you were racing the car it would be a little different and some cooling holes might serve a purpose in the length of a race. Just like all the vendors on e-bay selling drilled disks... its all for looks and money, or the factory would do it that way.