Well, obviously Coppersmith can be viable, if
- You can draw a lot of coppers (or cards in general)
- You can play multiple Coppersmiths (also King, Throne...)
- You have sources of +Buy to spend your money
It can be a nice payload for engines. The bad thing is, most engines don't like Coppers, so it's very likely that you want to thin them first, get your engine going, buy the Coppers back later.