For this reason, it's extremely important that you make using the kill switch a part of your daily routine. Always use the kill switch to turn the bike off when you reach your destination, and always use it to turn the bike on again after you've turned on the ignition. This daily use keeps the contacts of the switch clean and also (much more importantly) gets you in the habit of being able to hit the switch on reflex alone. This is the approach taught by the Motorcycle Safety Foundation's rider courses, and is very much endorsed by Modern Vespa.
Q: Why can't I just use the ignition switch? It's just as fast.
No, it isn't. For one, you've got to take your hand off the bars to turn off the bike with the ignition key. For another, keys aren't in the same place on all bikes. The kill switch is always on the throttle-hand control pod, easily reachable by your right thumb.
Let's put it another way: What if the brake was mounted on the inside of the legshield, and you had to reach down to hit the brake? Do you think your ability to stop the bike would be as fast as if the brake was mounted sensibly on the bars?
So why not the kill switch?
Q: Why did my dealer tell me not to use the kill switch?
Your dealer is wrong, period. In all likelihood, your dealer told you not to use the kill switch because they're afraid you'll forget to flip it back, and then wonder why your bike doesn't start. In other words, they assume you're an idiot.
In fact, if you don't make the kill switch a regular part of your daily routine, you will almost certainly find yourself sitting there one day, pressing the starter button, wondering why the bike won't start. This might be because some random stranger flipped the switch while your bike was parked, or because the seat bumped the switch, or maybe you just flipped it inadvertently. By using the kill switch every single time you shut off the bike, you'll never be at a loss for what to do because it will be part of your daily routine.
Q: I was told I would wear out the switch if I use it regularly
That's absolutely false. Electrical switches -- especially ones exposed to the elements -- are far more likely to corrode and fail with disuse. Regular use will keep the contacts clean and free of corrosion.
Q: I live in the UK and my riding instructor told me not to use the kill switch, ever. Is that true?
If you live in the UK, then yes. You should never, ever use the kill switch, because they are clearly made differently than they are made everywhere else in the world and prone to sudden failure for no apparent reason.