Valet switch is much like a valet key... it won't let people enter the glovebox or boot. So when a valet parks your car, they can start it, drive it, but not swipe anything in your boot and glovebox. (prolly not applicable for Skylines... no lock on the glovebox)
I agree that everyone should have a top-notch alarm. I read so many posts on SAU about people getting their cars swiped... spend money on an alarm before upgrades and neon lights or whatever tickles your fancy.
If everyone had a decent alarm on their Skyline, I recken most theives would stop bothering to try, and move on to other desireable performance cars.
I have two mates with stories from the end of last year.
First guy went overseas, got back to find his GTiR missing a bonnet, and the door had been jimmy'd open. Wires cut etc etc. The theives couldn't start his car due to the immobiliser. No alarm though, so they simply took what they could.
Second guy has an R31 with an RB20DET in it. Woke up in the morning to find it down the road, door jimmy'd open and the ignition barrel slaughtered. The theives couldn't start it. Not sure why it was down the road.. either to hide the starter motor noise, or to try and hill-start it. The ignition immobiliser saved his car. Again, no alarm siren.
In both cases, the immobiliser saved their cars, and they have since had everything repaired, except for a Pulsar bonnet to replace the GTiR bonnet. Sure, theives can tow your car, but there are alarms with tilt-sensors and shock sensors. Park behind another car, and towing becomes a problem. Either way, we need to address every possibility.
I have a 3-point immobiliser, with shock sensor, glass break sensor, and all doors/bonnet/boot sensor. If you try and take a wheel-nut off, the alarm will go off.
I test the alarm by kicking a tyre... it sounds off.
I arm the car then use the key to open a door or boot, it goes off.
Be safe, people! =-]