I'm surprised there haven't been some comparisons to the vintage car market here; a lot of the same refrains are heard. "Don't restore that car, it's a survivor, sell it or drive it as is!" "It's patina!"
I agree with not doing a restoration if a machine is in a playable condition, and further play won't degrade the machine further (i.e. if it's got an actively flaking playfield, something needs to be done before playing it a lot), a case can be made for leaving it as a "survivor".
But if it's completely shot from a mechanical, playfield or cabinet perspective, it's not worth much in that state (in my opinion). Doing a restoration would result in a playable machine that someone who doesn't care about owning a survivor (like ME, for example ) could enjoy probably for the rest of his/her life. So, in that case, do it!
Going back to the car analogy: One thing that could be done in a pin resto where things like backglasses and/or playfields are replaced, is to include the original parts in the sale (or keep them if you're keeping the machine itself). That's what the car guys do when they replace wheels or other parts on a car.
Myself, if someone made reproduction parts to build an entire machine from nothing except those parts (such that it contained no original parts), but the result looked authentic, played authentic, sounded authentic etc., I'd totally be on board with that. Many of you would probably disagree with me in the strongest possible terms.