Well, in theory a smart thief could bring bypass setup's and bypass all of these switch's. They could also bring a flatbed and just load it on and haul it off.
The primary purpose of these switchs is to make it annoyingly difficult, at best, for a thief to be able to rip the car off, quickly. If their are several different types of cutoff's, then the thief has to try to figure out what or where something is switched out and then attempt to bypass that circuit. If the thief makes a guess and it still doesn't work, then he has to try to figure out if their is something else, what that something else is, how to bypass that something else and then try to bypass that something else.
By the time you get several devices, the car gets to where it's just plain not worth the hassle and they will go find a different, less high profile, time consuming issue, to rip it off. It's really more of a question of making it enough of a hassle that your car isn't worth it. It could still be stolen, no question.
wires are a lot cheaper than a flat bed. its not theory that a smart theif would do it. its fact. they do it. the amateurs do not. the bottom line is they give up for the night in 5 min if they cant get the car started.
EDIT: upon re-reading my post i think i didnt make it clear that kill switches are important and do help against amateurs, which are what most theives are. however my main point is that they should not be the single line of defense.
don't waste your time or time will waste you
a few people on my custom truck forum hid several airhorns inside thier trucks and had the doors lock and stay locked if the alarm is going off(and the airhorns) poor bastage that trys to steal thier trucks...but of course if someone wants somehitng bad enough thier going to get it...thats why when you install an alarm hide the brain part of it high up behind ur dash as they cant see it and reach under the dash and yank the plug out. disquise the wiring. punt under the hood stuff where you cant reach it from underneath (the siren wires especially) and put a trigger switch on the hood, once it pops the alarm goes off. and make the alarm loud as possible (relays with airhorns are wonderful) as to wake you/anyone around/cause major attention in a parking lot.
I hang out on Team-Integra.net alot and since the Integra is well within the top 20 most stolen cars, they have several security mods you can do. Check out the article below. It does a nice explaination of how a Honda/Acura main relay works and how to connect kill switches.