I am a brake technicion, and I work for Brembo Its to do with the pad being soft enough to "mould" into the grinds you have made. If you have a pad which can do this (like a rim brake) it will be loads better because the total braking surface is increased. But on a disc (with hard pads) they will not "mould" into the grinds so the total braking surface is decreased, therefore decreasing the overall firction, and reducing the braking power. Also there is a good reason why every car disc is smooth.