If you keep the phone turned off when you're not making calls and call from random locations in some big city, the GPS location data won't be very useful. For example, "this phone was used to make calls from a few scattered parks in Washington DC" doesn't really help to track down who leaked some item of classified information. Using the phone from your house or workplace even once could be a dead giveaway, so if you're security-conscious enough to want to have an untraceable phone, you would have to use it in the way I described.