well- actually the cross drilling is to draw any pressure buildup or brake dust or for that matter anything that gets inbetween the rotor and the pad out from inbetween and as a secondary effect they aid in cooling- but the primary design intent is to evacuate anything that would impeed optimum surface area contact- the slotted rotors are the same idea- only they strive to retain maximum rotor area for more braking mass and subsequent heat absorbption.
even in OEM sizes these rotors help a lot - but only in situations where pads can beguin to float or not make good contact- this is usually caused by repeated high speed stopping - so if you dont drive hard and fast - you dont need slotted or drilled rotors-
It is true that you cant turn cross drilled rotors- well you can but then you have to re- radius and peen each hole- yeha basically by the labor cost its cheaper to get new ones- you can if you do it right turn slotted rotors but you have to re radius and peen the edges of the slots- which is hard but can be done resonably cheaply- that is assuming of course you know what your doing.
