I'll try not to write a novel on this, as I'm no expert at all, but this is a subject I've broached with a number of other people that are in the know, and I've done a fair amount of research myself on this subject.
Cross drilling/slotting of rotors on street-driven cars don't add too much apart from looks. This is especially true given today's pad compounds that are routinely used on street and semi-track cars. Notice I didn't mention motorcycles, F1 racers or anything else; I'm talking about this application on streetable coupes/sedans that will sometimes see track use. In short, Contours and other "sport" sedans out there...
Longevity
Most blank rotors that are drilled after they've been heat treated (i.e., placed in vats of sand to help them cool off evenly after being formed from the factory) will display a tendancy to form stress cracks or fractures due to the uneven heating and cooling placed upon the rotor. Potentially, all it could take is a couple of splases through puddles, staying on the breaks at a light after hitting and staying on them hard, etc. There is NO argument that longevity is adversely affected by going with a cross-drilled or slotted arrangement. This goes for pads, too as the slots and holes will eat your pads up faster than a blank rotor would.
If you can get a set that have been formed from the factory (Brembo does this, I think) as cross-drilled or have gone through some sort of cryo-treatment (Frozen Rotors or the like) to help strengthen the rotor, then I wouldn't be adverse to recommending them, as long there exists the understanding that you may be purchasing new pads/rotors MUCH sooner than you would otherwise...
Again, understand that going with slotted/cross-drilled in the majority of circumstances is going to shorten your rotor life.
Stopping Power
I've had folks tell me that their cross-drilled rotors have drastically decreased their stopping distances. I doubt this by itself has done this for a number of reasons:
1) Decrease of Surface Area - Less area that the pads can exert their force from the pistons. It's physics and it doesn't change to justify the amount you just spent on those fancy looking rotors. The pads and rotors they've just replaced are usually shot to hell, so the new pads and/or brake kit is usually behind 95% of the increased stopping power that they notice. Again, this is comparing blanks and cross-drilled/slotted rotors of the EXACT same size. If you're going from blank 11.5" rotors to cross-drilled 13" rotors, then things are going to be different. Hell, you may not even get the rotors up to full operating temp (depends on the type of pads you are using). I've heard of folks DRASTICALLY increasing their rotor size and putting track pads on and try to live with them for daily use. The first time they made an emergency stop in the cold sold them on going back to street pads...
2) More Pistions - This is where I get a little out of depth, but the more pistons you have exerting force across the area of the pad directly correlates to the increase of the amount of pressure put on the rotors. Someone with a better physics or hydraulics background please step in and explain it better, as I'm sure I've taken a rather hacksaw approach at this one...
3) Operating Temp - Another area I'm shaky on, but the bigger the rotor, the better the heat sink it is. Running bigger rotors that are cross-drilled can adversely affect your breaking distances, due to the cooling effect that it has upon the rotors. Brakes have a temp window that they operate best in. Too cool, the don't function as well. Too hot; well, everyone knows about brake fade... The deal here is that by going with MUCH larger rotors, you are changing that window. Whether good or bad, it depends on your driving and pad compound. I'm assuming that we are talking about street use here, so going with a really grabby track compound that has to hit higher than normal temps to function optimally and 2" oversize rotors that are cross-drilled probably isn't a good idea.
In short, I'd tend to agree that for 4-door sedans such as ours, cross-drilling/slotting is pretty much for looks. I've heard the wet-weather argument that they perform better in rain, but unless you're in Washington or Oregon, I'd try to find another excuse to justify them.
Performance-wise, I just don't think it lives up to the hype. The do look great, though.
