I'd boil it down as the tank is successful, but the owner doesn't necessarily understand why.Lastly, IF a person removes parasites from their tank, and their animals will no longer be exposed to those items, there would be no reason to keep exposing them right?
Personally I still think it's a feature of mature tanks to be disease resistant because they have a mature eco system full of things the disease organism has to compete with rather than being a sterile environment where they can proliferate unchecked.