An interview on the New Scientist website discusses the problems of art restoration and the restoration of Michelangelo's David specifically. One part of the interview particularly stood out for me:
A work of art is pretty much like a human being. We all get battered, we break bones, they mend, we go and get some disease, we get cured, and then we die. There's an organic life to a work of art, too. It accumulates experience as humans do, and those experiences shape it. Once you've understood that, the idea of going back to the original seems pointless, even if it were possible.
I've felt something of the same thing when I've seen restorations done on some of the major buildings in Paris. One image that struck me was the cleaning of a building along the Seine. Half the building was a dirty marble color, while the other was an abnormally bright, teeth-bleached white. The dirty, unrestored side was majestic and old, a part of France's history. The other side hurt my eyes.