It's a very painful and deadly disease, you give the pill to "help" prevent it, you give the test to MAKE SURE that they don't have it, as you say, they can still get it while taking the pill, and then there are all those owners who skip a month. The sooner the vet catches it, the better the prognosis. Don't be fooled by the fact that very few dogs in your area are getting it, perhaps most of them are on heartworm preventatives? Here's some more info that explains the why's.
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Before starting a preventive program, all dogs should be tested for heartworms. Giving preventives to dogs that have adult heartworm infection can be harmful or even fatal to the pet.
Adult heartworms produce millions of microscopic "baby" heartworms (called microfilaria) into the bloodstream. When you give a monthly heartworm preventive to a dog with circulating microfilaria, this can cause the sudden death of microfilaria, triggering a shock-type reaction. Even if your dog does not have this type of reaction, heartworm preventives do not kill the adult heartworms (although they may shorten the worms' life expectancy). This means an infected dog will remain infected with adult heartworms.
Unfortunately, as long as a pet remains infected, heartworm disease will progress and damage the heart and lungs, which can lead to life threatening problems. Giving heartworm preventives to heartworm-positive dogs can mislead an owner into thinking everything is all right, while within a pet, heartworm disease is worsening.American Heartworm Society | FAQs |