> There are frameworks and libraries that handle 100% of clipboard OS specifics
They're sufficient in many cases, but you'll still sometimes need the control of working with COM/etc. directly, and those libraries don't fully save you from platform-specific bugs (e.g: https://github.com/glfw/glfw/issues/2644).
> the app in question has no use for system clipboard in the first place
What do you expect to happen when you copy some text from an external editor into a text field?
They're sufficient in many cases, but you'll still sometimes need the control of working with COM/etc. directly, and those libraries don't fully save you from platform-specific bugs (e.g: https://github.com/glfw/glfw/issues/2644).
> the app in question has no use for system clipboard in the first place
What do you expect to happen when you copy some text from an external editor into a text field?