Scenario #4: BG Shoots someone (mugging gone bad, spat between criminals, whatever) and takes off. Victim falls to the ground, but instead of bleeding out overnight (since no one called 911) is still alive when cops show up and activate EMS.
It's an expensive system, but it does work pretty much as designed, can help, and is an order of magnatude less stupid then many things NYC has spent tax dollars on. Meh.