From my understanding, the reason the slow kill option was used is that when an adult heartworm dies, it releases a toxin. If too many adult heartworms are killed at once, the toxin issue is too great for the dog, and that is what kills the dog.

Growing up in the MS Delta my entire life, and having dogs there for most of that time, I can attest that if you do NOT give your dogs ivomec year round, your dogs WILL become heartworm positive in no time flat. That area of the country may reach the sub-freezing mark a week out of the year. While it does have it's relatively mild cold times, it is never enough to decimate the god awful mosquito population that resides there due to all the farming and irrigation.