I believe that date-coding reproduction parts is misleading and dishonest. It's too easy to make artificially inflate the value of a car doing this type of thing. What about the survivor class vehicles that are at every major car show? It totally defeats the purpose of this class. And although a trained eye can tell the difference if compared to an original, not many people have that trained eye, or the occasion to compare to original. Too easy to mislead a buyer.
On the other side, I can see where someone wanting to accurately restore a vehicle would feel justified in using such items when NOS is non-existant or too expensive.
There's also a real gray area here. What about people like Jay Leno, who pay to have rare, one-of-a-kind vehicles restored? Many times parts are made by hand, to factory specs, or from photographs. They claim that these cars are restored authentic vehicles, yet the parts should be considered reproduction since they are not the original parts from the original maker.
I just think that if reproduction parts are going to be date-coded, then they should also carry another stamping, on some hidden area, that denotes it is a reproduction.