No I agree, it was useful for us only because we had it set to send the current location every x minutes (whatever it was) along with the battery info, again in the event that we lost contact with the app we could look at the prior update and see what the battery strength was at the time.
Because this is presence-based, I would just include it in the update and make it another data element of that update. Again we used it more for “We haven’t received a location update from this user in X minutes and at the time their phone was < 10%. Notify the first person on their contact list that the phone may have died.” That was after we would ping the phone and attempt to get back a real-time update, which again would fail if the phone was dead.
I can’t say for sure I can come up with a piston I would use this for, at least right now.