Originally Posted by PowerDDanger

Considering that most rotors made of steel, any contact with water eventually causes rust especially if they sit on the lot for a while or during transport. Powdercoating is okay for the brake calipers because its not a part that is considered moving, it mostly just holds the break pad in place. The rotor on the other hand is moving as quickly as the wheel, put under a lot of pressure and heat during stopping. If a piece of powerdercoating gets stuck in front of a break pad it causes serious danger because the break pad does not contact the rotor as efficiently.

Check out this animation, the red is the caliper and the plate is the rotor:
http://www.youtube.com/watch?v=ol-wDa6kAgQ