My wife lost her phone a month or so ago. I asked Alltel to use the GPS feature to find the general location of the phone; they told me that they are not able to track the location of a phone. If that is true why would I want to turn the GPS tracking feature on. (It is my understanding that the tracking is always on for 911.) BTW one of her friends did find the phone and returned it.
You need a court order to do that. The reason you would want it on though is if you wanted to use a GPS program on your phone.
To give you further info: The GPS capability on phones is almost always, by default, set to e911 only. This way your battery life is extended. It only uses that function when you call 911. So if you don't have the GPS set to Always On then the phone can't be tracked UNLESS the person who has it dials 911. It is possible that the signal could be triangulated, but that's an entire different set of technology we are talking about.