If I was stealing an iPhone, (or an iPad for that matter) the first thing I'd do would be turning the phone off so it couldn't be located with find my iPhone. This could easily be fixed if you had to put in your pin to turn it off, and the thief simply had to wait for the phone to die in order for it to be off the grid. This way, if someone stole my phone, it would be active and traceable till it ran out of battery, or the thief would have to smash it to get it off, which would be, well, less of an option. 
I can't see a downside to having this feature implemented?
I can't see a downside to having this feature implemented?