Originally Posted by GJeep

Yes, that's what I know. Mud is soil grains and water. The grains are not soluble, are pretty hard, and can easily scratch the disc. Dry grains fly off, mud sticks. Slots or holes allow mud to get inside, where there's supposed to be free air flow. Removing dry mud from in there, is an unnecessary burden. Sooner or later, even in dry regions, we cross some mud.