Find my iPhone is really a mess. It needs these two fixes.
1) It should have been integrated into the ios like the weather app or any of the others, that way it could not be removed and it should have no Off mode, that should be something that can only be done on the net when you log into the account.
2) It should have worked with the phones serial number or CID so that once you register it the phone remains trackable by you and only you, unless you decide to login into Apple and release the number when your selling or giving away the phone.
If those two things where done it would not matter what the thief did because sooner or later he will have a Sim in the phone or wifi connected and Pow you know exactly where he is and if you can't get your phone back because he's in Nigeria, etc. You will still be able to remotely wipe his phone every week or so just to piss him off.