Rotors are cross drilled to vent gases out from between the pad and the rotor. As the pad "burns" away, gasses are produced between the two. Under extreme heat and hard braking, those gases on a solid rotor can push the pad back and off the rotor causing a loss of braking power. The slots wipe the face of the pad clean so you have a constant fresh surface. THAT is why they're slotted and drilled. The vents sandwiched in the middle of rotor (around the circumference) are for cool. Sure, the drilled holes assist a lil bit in cooling but that isn't the original intention. Just a lil FYI