I can see some wise reasoning for this. Restoration very often involves the removal or alteration of an original part, surface, or finish. For example, let's say a chunk of wood has been broken off of the corner of a phonograph cabinet base molding. To repair/restore it, you would possibly even out the fractured surface to prepare it for gluing a new piece of wood. This will entail the further removal of original wood and surface finish. When the new wood piece is glued in place, (incorporating an unoriginal wood joint), it will then need to dressed down and shaped to match the original molding form, which will necessitate further removal of original finish when blending the new & old molding. Staining and finishing the new piece will add an unoriginal, refinished section to the cabinet. When it's all done, it may look fantastic, but when you consider what's left of the original piece, there is less there than what you started with. There's also the question of what level of workmanship can the museum expect from a given restorer.I can definitely say that there is a conservative museum mindset in which conservation of an artifact as is trumps any restoration to a functional state.
Also in this museum mindset, the use of artifacts is generally forbidden, unless they are common, disposable, and replaceable.
I realize I'm stating here what a lot of us already know. However, I often find it necessary to remind myself of this mindset when seeing a damaged object on display or when I purchase a less that perfect example of a particular item. The "fixer" in me sometimes needs taming.
