For me it's all about layers of protection. I have a $25 china generic alarm from ebay, wired into the central locking to give me keyless entry and an alarm if someone opens the door while armed. That does two things for me. Firstly, the keyless entry means I actually bother locking my doors (I really can't be stuffed using a key). Secondly, the alarm is intended to scare away opportunistic thieves who get into the vehicle before they destroy my ignition. I don't use the immobilizer features of the alarm however. I have a totally separate hidden kill switch that allows the starter to crank but kills the fuel supply. I then have a GPS tracker in the form of an old Android phone embedded in the chassis. The idea is each system is independent and acts as a backup for the others, roughly like this:
1. Alarm to stop brute force thieves from damaging my vehicle too much before I scare them off.
2. Kill switch to make driving off with the car more difficult if the alarm doesn't work.
3. GPS tracker if none of the above work, or they bring a tow truck.
Most importantly, since all three of these systems are totally independent, unique to my vehicle, and not at all obvious even if you're sitting in it, I'm pretty confident it'll stop 99.9% of attempts at theft, or at least allow me to track the vehicle in that event.