Coppersmith is a terminal action, therefore you have to design your deck to accomodate it. (By terminal action, I mean it is an action that does not give +Actions). As others have noted, Coppersmith is a card used to give you an early start. By managing your deck, you can extend that advantage well into the mid game.
Here is a chart showing how 1 coppersmith can accelerate your early game. I've compared it to Smithy, and "Big Money" (A base strategy of buying only silver/gold/provinces which is actually fairly strong)
Chance of Having (once during the two turns)
Turn 1-2 purchases $6 $7 $8 Notes
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Coppersmith+Treasure 50.91% 0.00% 21.21% # 1 in 792 that 2 golds can be bought
Smithy+Silver 42.60% 19.39% 2.12% #Not possible with a 5/2 copper split
2 Silver 33.59% 8.84% 0.00% #Not possible with a 5/2 copper split
This shows a few things
- you have a vastly higher chance of affording a province.
- The coppersmith gives you a 70% higher chance of being able to afford a gold than "Big Money"
- The coppersmith works with a 5/2 copper split on the first 2 turns, while the other two do not.
- Coppersmith will not give you possibly annoying $5 and $7 hands.
Is buying a province on turn 3 a good move? Maybe not, but it could be. My point is that if you buy coppersmith, you stand a higher chance of getting an early gold. This advantage will carry over to future turns. Coppersmith will help you hit the financial high points of $6 and $8 more often, while avoiding $5 and especially $7.
Every set of 10 cards in Dominion is different. Coppersmith won't always be a great card, but it can be used to great effect. Try playing it with wishing well!