Many people, especially in northern climates, will often have at least two cars: their pride and joy and their winter beater. Whether you drive a winter beater or a show car, your car will benefit from being undercoated with a rust-proofing material. Undercoating comes in a variety of forms and some forms protect better than others. Generally, any undercoating is better than none. The only cars not recommended for undercoating are show cars that are trailered to judging events and are never exposed to the weather.
Eventually, road salt catches up with winter-driven cars and, for some, no amount of body work can save them. For these winter beaters, a good oil spray (really any oil spray) really helps to squeeze a few more years of cheap service from these cars. Without more than anecdotal evidence, I will go out on a limb to say that the cost of even the cheapest annual undercoating job will pay back with at least an extra year of service. Usually, even one application early in the life of the car will pay back will may years of additional service.
Back in 1977, my Dad bought a brand new Pontiac Parisienne and he knew that it would last longer with undercoating. Not wanting to pay the seemingly exorbitant dealer price for this service, he decided to do this himself. He had a friend with a hoist and a compressor so he sprayed the under side of his car with some undercoating he bought from Canadian Tire. Thirty years later, the underside of this car is still in excellent condition. However, since he didn't have any wands to get inside the doors and the hood, these parts are rotten and it's probably not worth saving this car. Too bad, because the car still runs and drives great.
For your pride and joy, a good oil spray will help to protect it from the elements, even if you don't drive it on salted roads or when its raining. There is a potential for corrosion to occur just from the moisture in the air. If you only do a cheap undercoating job once in your car's lifetime, that investment will most certainly save you much more in rust repair jobs years down the road.
Professionally applied undercoating often comes with some sort of warranty. The warranty is there to protect the shop from your claim. If the warranty was there to protect you, it wouldn't have any exclusions and the shop would just take care of you because you had the receipt. When you hire someone to undercoat your car, find out what they are putting on and how they are doing it. Remember buy the undercoating service, don't buy the warranty!