The phone starts with the aluminum unibody (rear case), and every single component is then placed and layered into it. Replacing the rear case literally involves removing every component from the phone. Add that to the fact that an OEM quality rear case with a sapphire camera lens cover won't be cheap, and you've got yourself a time-consuming, risky, and costly repair. Spend $30 or $40 on a good case if the scratches are going to bother you that much.