With missing you opt into three valued logic (like you gave as an example). I never want that because it would hide bugs in my code, so I use nothing.
The whole Missing story could kind of be a package (it used to be) with the exception that it is used in some cases like find on arrays which might have missing values and you would need to do type piracy to support that. Nothing on the other hand is quite a bit more fundamental and is ingrained much more deeply in the language (even being a part of the iteration protocol) and being the implicit return value.