My vet gives me the lecture while pushing me to buy pills in bulk and switch to a flea and tick preventative.
I'm giving them the benefit of the doubt and assuming they give that lecture because people do board their dogs or leave their dogs outside, or live in swampier areas than I do. And in which case, I do agree that some dogs need to be covered year round.
Yep. Heartworm preventive usually do more than just heartworm. Intestinal parasites can be an issue in the months when heartworm isn't. And if your dog is ever around other dogs-the groomer, the boarder, the vet-there is a definite risk of fleas. Everyone has to decide what they are willing to risk.
I give Interceptor year round for the reasons IowaGold states. I also use Frontline Plus year round for the reason stated plus the fact that we track (and there were still ticks this week) and do field work.