a lot of display backs just for the sake of it when the finish isnt particularly good. to be honest i dont think i own many watches without display backs(vintage pieces excepted).
i do like the idea of Omega putting display backs on speedmasters even making aftermarket backs for the early pieces.
so yeah, if its worth looking at then put glass in it. if it is a bog standard unmodified movement stick metal on the back.
just me humble opinion
Graham