if a thief cuts the battery wires? how will they start the car? if they re-engage the wires, the alarm WILL go off. why need a battery back-up?

 

They tow or can take off the parts they want - or take the time to find the siren/horn disconnect and then hook the battery back up.

 

Most thieves don't go for the car. They are just there to get something quick (i.e. seats, sound system, tower bars, intake, knob, rims, etc) -- these are the "small thieves.

The big thieves will, of course, tow your car if they really want the car (in that case they can also cut the wire too). Towing is generally hard if you parked the in a parking lot (since it'll look weird having a tow truck towing a car in a movie theater lot). Cars generally get towed if they get parked on the streets ...

Backup battery system or siren with backup battery won't save you, but hopefully that'll ask the thieves to move on to another car instead of yours ...