This is not true. A battery backup siren is a horn with its own battery built-in. You can't shut it off by disconnecting it.

I believe in the laws of probability. If you have a stock alarm, the odds are good that a would-be thief won't be successful If this wasn't true, there would be no reason for insurance discounts.

I've never seen any statistics on the success of car alarms, but I'd be inclined to believe that as you increase the level of sophisitication of an alarm system, there are a smaller number of thieves that are capable and willing to complete their mission. A noisemaker is plenty of deterrent to keep most (true, not all) cars from losing their radio antenna, let alone their seats.

And LoJack isn't a deterrant. It's an after-the-fact thief catcher.