Okay. My point is that you cannot know (unless there are no circular
dependencies) *when* a destructor is called by the engine;
The benefit of non-public visibility isn't when it's called, but how many times it's called. If you can declare your destructor non-public you can be confident it'll only be called once per instance (By the engine)
Or is there a scenario where the engine will call a destructor more than once on the same instance?
Destructors should (IMHO) be public. Not necessarily because they can be called, but classes with destructors hint at underlying behavior when destructed. For performance, you might want to defer that by retaining a reference. If a class has a hidden destructor, you have to go read the code to find it.
Wouldn't you have to read the code to see if it had a public destructor too?