What? Google it...they have drilled and slotted rotors for a reason I didn't think I had to explain. I'm not sure what your saying. They help keep the pads cooled down which doesn't burn them up as fast and helps it stop faster. And it's not rougher it stops just as smooth as OEM.
To me, "Helps the life of the brake pads" looks like you're saying it increases the life of the pads, as in it increases their mileage. Heat buildup is more of a short term thing. Yes, heat will damage pads, but street driving really won't cause that kind of damage.
The marginal gains from the extra heat dissipation (which is debatable) will be easily outweighed by the extra wear on the pads from the choppy surface. To the argument about pads and rotors being too stiff to dip into the slots,
everything flexes. Further, rotors do wear down, which means the pads are eating into them.
As for the heat dissipation, it doesn't work as well as you think. You stop the car by converting rotational energy into heat by way of friction. While yes, drilled/slotted rotors offer greater surface area, they also reduce rotor mass, therefore reducing heat capacity. Since you still produce the same amount of heat for a given stop, the extra heat has to go somewhere. The rotor will reach a higher temperature, as will the rest of the braking components in the wheel. It's a tradeoff
About the only benefit you have over blanks is a slotted rotor because the groove gives the brake dust and water or whatever is on the rotor surface somewhere to go.
Also weight reduction, reducing both the inertia of the vehicle and the rotational inertia of the rotor. Some professional race teams run drilled and slotted
As for cracking, I believe there was one Porsche that had the holes made in the casting process, rather than post-cast drilling, to avoid cutting into the metal's "grain"
