Eh..guys. This theory just struck me in the head when i was taking bath yesterday.
Anyone with my age or older than me would remember this. When i was a child, my grandmother or grandfather would teach me to poke holes in a bowl of rice to cool it down faster or make a hole in the center to cool the rice faster. I do think that this theory applies to the brake too.
2 steel block that has a same mass and dimension but 1 is drilled with holes which one would cool down faster ? the one with holes rite ?
Why ? because more heat can be transfer to the ambient air. Heat can be transfered from the surface to the air and heat inside the disc can be transfer to the holes and air thus provide better cooling. If only a blank rotor heat can be transfer to the ambient air too but heat transfer only happens on the dics surface heat inside the disc will have to wait the surface to cool down to transfer more heat to the surface.
there are 4 principles in heat transfer.
one of the principle is (Heat is ready to flow or pass to anyhing that has less heat.)
So what i think is the holes in the disc surface is to remove heat at a more faster rate because ambient air is definitely cooler than the temp created by the braking force.
If a brake temp is too high, brake fading will happen and it may cause the dics to bent due to the high temp. We know that metal is hard to bent when its cold but when its heated to a certain temp it could be bent much easy.
Another thing is with cross drilled or slotted, we can increase the working temp of the brake system. With faster heat dissipation rate we can get a better or higher working temp.
Just like why some cars install oil cooler. oil cooler will keep the oil cool so that you can rev much higher and the oil can sustain a more severe driving condition without overheating. If the engine temp is too high, efficiency will drop. Just like our brake, if its too hot it's efficiency will decrease too.
Just my own 0.0002 opinion only. No hard feeling guys.