When Did Cleaning Coins Become Such A Big Deal?

At the risk of this dead horse necro thread - I was watching Antique's Roadshow the other day, and many of the owners of high value items were told, "in this condition you would realize x at auction, however if restored you would realize a much higher value". I find it interesting that unlike most other old things, restoring coins doesn't add value and generally detracts from it. There was an old steamer trunk that reminded me of a nicely circulated coin - in great condition with original patina and some well earned dings and lustre loss and the owner was told to restore it. Seemed wrong to me.