Until the theif figures out how to disable that part of the alarm system.
I like the fuel pump relay thing, because you can stick the relay in your pocket and walk away.
If someone does get in, they need to know how to diagnose a missing relay, know where to look for the relay, and know how to jumper the relay to start the car.
Most Mustang enthusiasts on this site would need a wiring diagram to do this, so I doubt the average theif will put in that effort...