Book conservation involves a tough decision making regarding usability and long term preservation handling.
Sometimes we are dealing with fragmentology, or disjecta membra (fragments of written supports are re-used as part of a binding, with a diverse purpose from the first use) whereas in other cases the purpose is the same, and the whole binding is being recycled to bind a different book.
Incunabula and manuscripts with laced-case and limp vellum bindings are studied.
Not uncommon to find waste material as part of a binding (parchment manuscripts, for instance), but quite much rare when the previous and original binding is part of the boards layers of the current recycled binding!
The outer and visible binding belonged first to a former book, and the hidden binding, to the current, one... crazy!