I think what happens is the drilled hole end up being a weak spot in the surface. I have seen rotors with crack radiating out from drilled holes. Never seen any damage or associated problems with slots. If you want more cooling of the rotor/pads/calipers/wheel bearings, I would vent some air directly onto the rotor. That's what I did after having an extreme overheat of one side on a track day. Easy enough to do.