Quote: But sometimes grease fills it in or it get abraded away by mint employees
It can get abraded away but it is not likely to disappear due to a grease fill because the letters are RAISED on the die. In order to lose them to grease fill you would have to lose the whole base of the bust.
I can still see the affected area. Why do these disappear? The VDB is incuse on the coin, raised on the die. So when die polishing happens it get polished off the die. The weak or even missing totally. No premium for these. I remember a seller on eBay that used to charge medium money for these, but he finally figured out it was not a premium issue. (Just a die event)