I got the elecronic rust protection unit from the dealer. Cost around $1100 one time, checkups are free with regular service.
Since you paid 1100 I hope I am wrong but there have been lots of independent testing that shows they do not work, and their science is also broken (completing the circuit for electrolysis, immersion, etc.)...
For the OPs question:
Most cars today are made with galvanized or at least zinc coated steel. They are all e-coated (electro coat epoxy primer) that seals it up really good. To rust you need to break through these coatings. The biggest problems are design flaws (think older civics rotting right behind the rear tire at the bumper seam) that trap moisture in pockets or seams and "road salt" and chips that break through the coatings.
For the OP, how long do you plan on keeping the car and what kind of car is it? Lets say you plan on keeping it for 4 years, well it is pretty unlikely it will rust in 4 years so all you did is spend money for someone else's benefit. Basically the next guy is not going to pay you more for it because it was rust proofed, for a car that would not be rusty anyways.
Now lets say you plan on keeping it for 10 years. Now some cars will be rusty in 10 (our old Hyundai was a great car but it had holes around the wheel wells at 10 years) so it may be worth doing. Some will not, our 2002 Saab has never been rustproofed (other than what came from the factory) and there is no rust on it yet, even where it has been scratched and chipped through the primer.
If you intend on keeping the car for a very long time it may be worth doing, question is in 10 years will it get you the cost of the treatment more in resale (over a grand more) for no rust? Maybe, Maybe not. But at least it will look good while you are driving in those later years.
Take a look on the road for like cars from the same brand that are a decade or so old and see what they look like. Not a guarantee (lots could have changed in the manufacturing process over those 10 years) but it will give you an idea of how it will hold up.
I would stay away from anything at the dealer for rust proofing, they are using better stuff now but in the past they use to spray rubberized crap that would peel trap moisture and rot floors and brake lines. I would be skeptical that anything they spray now will last 10 years... Not only that, how will it seal up a new chip that happens a decade after they applied it? How will it neutralize the salt trapped in a design flaw years after it was applied?
The Krown and other guys in that league spray the car with a chemical every year, this will get into those design flaw places and help on those scratches. This assumes the guy knows what he is doing. I have seen great results but you do have to keep doing back. Many people do not like the fact they drill holes to reach pockets.
One last bit of advice, think back to high-school chemistry. Oxidation rate is temperature dependent, the warmer it is the faster the reaction. The worst thing you can do is drive through all the crap they spray on the roads to melt ice and then park your car in a heated garage, you added the chemicals for the reaction, then added the heat! Even if your garage is not heated or is just an underground parking lot it is still warmer than the temp in the driveway so it will still rot faster. Below zero storage when the crud is on the car is the best temp, that is your driveway, not your garage. Garages rot cars, it seems illogical unless you paid attention in high-school science...
