There are some idiots online that think slotted rotors are only for race cars.  Check out the pictures of the horrible and deep "record grooves" on my stock rotors.  The chances that the pads will line up perfectly with the groves every time is slim to none.  If they don't line up, then the braking force is drastically reduced due to the very small contact area on top of the grooves.  Slotted rotors wipe the pad so that they can't develop grooves. Flat pads keep the rotors flat and the braking performance up.