This means that in the event the thief turns your phone off, or if the battery is dead, the feature is all but useless. The good news is that Apple is looking into fixing this because according to a recently discovered patent, Apple seems to be exploring the idea of keeping the feature functional even if your iPhone were turned off/dead.
The patent describes how Apple could create a feature that would “periodically exits an unpowered state and transmits location data.” Basically every now and then, a portion of the device would be powered on, long enough for the location module to transmit its location. However we’re not sure if this will work on a battery that’s completely dead, or unless Apple plans to create some kind of reserve power just for this purpose.
In any case it’s just a patent which means that there’s no way of knowing if Apple ever plans on making it a reality, but it does sound like a pretty nifty feature that we’re sure many iPhone users would not mind having.