I did my own on the milling machine. There may be some performance advantages with degassing the pads and clearing away brake dust but it's mostly cosmetic for me.
We always used to drill the discs on our motorcycles to improve their performance in wet weather. On a bike the disc is right out there in the rain and the holes help get the water out from between the pads and the disc. From experience this really made a big difference on the bike. I've noticed that after washing the family car the brakes are a bit ordinary till the water is dispersed. I guess for normal driving the disc is pretty well out of the weather inside the wheel but it might make a difference in very wet conditions.
Cheers
