Like said before, I only switch it off if I need to have a good launch from a traffic light. There is also a small track around my parents house where I go to practise powerslides etc, of course it goes off then as well.
With bad weather I don't even think about turning it off. With the standard Potenza's I tried to launch on a wet surface with ESP off. When I changed to 2nd gear, wow... That was a real eye opener. The back of the car went from one side of the road all the way to the other, freaking dangerous! I just managed not to crash it