Benefits and drawbacks of drilled/slotted rotors, from the perspective of an automotive professional; I'm a professor of Automotive Technology at a junior college, a former Ford Service Training Instructor and field service rep, a former Ford Senior Master Technician, have some engineering background, and all around tinkerer of things that go.
Here's what I know to be accurate about drilled/slotted rotors; the depression features in the friction surface serve to increase effective surface area, which directly correlates into the components ability to surrender heat to the surrounding air. Under normal braking, this effect is negligible, as the mass of the rotor is capable of absorbing the heat from friction at a rate greater than the heat is generated. Under severe/extended braking, the rotor's mass will reach maximum heat saturation more quickly than it can be surrendered. By drilling/slotting the rotor, the surface area exposed to ambient air is increased, which promotes energy release from the rotor to the air. While drilling/slotting does somewhat reduce the mass, and therefore reduces the heat saturation point, if the holes/slots are correctly sized and spaced, the increase in heat transfer to ambient air is much greater than the loss of mass.
In severe braking applications, holes/slots will permit rapid venting of gasses generated by the friction material. These gasses are proven to build pressure between the friction material and surface, and push-back the friction material, reducing friction and degrading brake feel (spongy pedal/lever) during the outgas event. Holes/slots give the gasses an escape route, and help the friction material stay in contact with the surface.
In wet situations, solid rotors can maintain a thin film of water on the surface, reducing initial brake friction. A few hundred milliseconds after application, that film will be removed, but this a noticeable delay to most. Drilled/slotted rotors significantly reduce the effect of water on the surface during initial application.
Holes drilled completely thru the friction surface can promote cracking of the rotor IF it is heavily heat cycled. Most street driven vehicles will not see this type of heat loading. Dimpling or slotting removes the cracking failure, but does not add as much surface area, so the rate of heat transfer is reduced.
Drilling/slotting also contributes to some brake noise, primarily a "wire brush" type sound, that some find objectionable.
The benefits and drawbacks, as stated, are accurate to the best of my knowledge. The real discussion is as to whether or not the individual rider wants/needs the benefits, and are they willing to live with the drawbacks. Most of use don't really NEED drilled rotors, but they sure look cool!
