It depends on the quality of the restoration work and how much of the original components have to be restored. Essentially you don't want to have a piece that is completely damaged stay in a state of disrepair . It would be better to have it brought back to a beautiful functioning piece than to allow it to languish away forever. A professional who can replace the more simple components such as pin backs, jump rings, chain, stones, etc. by using vintage parts that are comparable or almost exact will not devalue the piece (in my opinion). I have (professionally) restored pieces myself for quite a number of years and received full value on them afterwards. My buyers were aware of what was done and preferred to have them wearable again. More difficult restorations such as repainting enamel also may not devalue the piece if done correctly and professionally. I am a professional porcelain artist who has restored enameling on many of the 1920's-30's pieces, again with the buyer knowing what was done, and received full value for the piece. For the ones who are purists and don't ever want anything restored, I respect your decision to keep a piece of jewelry in a damaged state if desired, but the value will be lost there regardless.