Bought two of them this year. One for my Austin, and one for the Suburban when I got it. Audiovox systems are my favorite, but Cobras are very good also. The Audiovox's were $150 installed, but they give them to me for $99 and I install them myself.
You're right about people ignoring alarms, but with the added security of a other things like kill switches, battery disconnects, removeable steering wheels, etc., the thief will get nervous and leave before figuring out what else he needs to do to get it running.
A garage is still the best deterrent. Out of sight, out of mind.

 

A car with even the best of security systems can be jacked if someone wants it bad enough. With these older cars, I could start it up with just a length of wire and a screwdriver, even if you have a "security" system, or hidden kill switch. A battery cut off switch would just require an extra jumper cable. Slow a thief down is all you can try to do.

The Super , Joe Pesci - YouTube

 

A battery kill switch can be jumpered, but it needs to be accessible, and you need to have a heavy enough cable to jumper it and carry starting load. I've had guys tell me that they could just clip a set of jumper cables across the battery switch, but the jumper cable handles would have to fit in a tight area to do so. They could also carry a set of wrenches and unbolt the cables from the battery switch, and connect them together, but they'd still have to get past the security system and it's ignition kill switch. If you jumper the coil to do so the alarm goes off. With the motion alarm part, they wont get the trunk open to jump the switch without setting off the alarm.
Security systems aren't foolproof, and neither are battery kill switches, but with enough good devices you can make a thief decide to go find an easier car to steal.

 

The goal is always to make the thief go to another car.
The more work yours is the less it is worth messing with.
Everything is steal-able.

I have been very happy with my cameras. I have motion cameras on the Camaro and C10.
Anyone around them and I wake up. Then decide to go hurt them or not.