Reason 1: Safety!
Most European brake rotors are designed for maximum performance while remaining as light weight as possible. Each rotor has a minimum thickness measurement stamped directly in the rotor itself. When you “ turn “ the rotor you are actually removing material from the rotor, which most often reduces the thickness below the minimum safe operating thickness. You run the risk of warping the rotor or even worse causing the rotor to crack! 
Reason 2: Noise!
When the brake rotor is turned, the new surface is not always smooth. Any imperfections in the rotor surface may cause accelerated wear on the brake pads and may result in brake squeal.
Reason 3: Cost!
For the price you may pay to have the old worn rotors removed, turned, then reinstalled on your vehicle you could spend a little more and have new matching parts installed. You run the risk of having to replace the same parts sooner, paying for the work to be performed twice.
