Having copper and silver might be interesting for immersion purposes. But unless Larian add items with prices that require the use of those coins - they will be just a cosmetic addition, that will only confuse some players, who don't know conversion rates.
Agreed, I don't see what this adds to the experience other than needless complication. The only game I can think of that did this was Temple of Elemental Evil and I never felt it really added anything to the game except slight frustration.
