Gamification is not one monolithic thing, so asking when it is appropriate is like asking when design is appropriate. To me, it's really an exploration of the fact that Game designers and behavioral scientists are asking many of the same questions. It's about taking inspiration from game design and behavioral science to influence voluntary user behavior.
Every app is designed for behavior change, intentionally or unintentionally. From an ethical perspective, I'm just trying to make sure that I'm intentional about helping the user to close the Intention-Behavior Gap.
If I know their intention and current behavior, I can influence them to make them more likely to close the gap. If I don't know their intention, then I can design the choice architecture so users reveal their intentions to me. Once I know those intentions, I can influence them to increase their odds of success.
So I'm not sure if there is a time when I'm not at least considering principles of behavioral influence in the design, whether I'm guiding behavior or listening to intentions.