historically, german car manufacturers like to use "thin" or "just right thicknesses" on their rotors. this saves weight and prevents chances of warping rotors over the course of a few pad changes... this goes for bmw, audi, mb, porsche as standard and inline with the performance angle...

so when your pads wear down, the sensor is used up.. that means you replace all 3 components at once. if you replaced before the sensor is used (before the dash light is triggered), then you can reuse the sensors. if you have the dash light on, then one or both of the sensors is worn also...

hth