If your library processes data from another language, you'll probably need to deal with the possibility that the library returns open ended enums.

I believe I've also seen this declaration for generated bindings for a JSON API that promises backwards compatibility for calls and basic functionality at least. Future versions may include more options, but the code will still compile fine against the older API.

I don't think it's a great tool to use everywhere, but there are edge cases where Rust's demand for exhaustive matches conflicts with the non-Rust world, and that's where stuff like this becomes hard to avoid.