Currency is only as good as the value that people put into it. Without backing, it is simply an inventory space taken up. I dare say that the bars that the coins are made of will see more value due to the fact that they are more readily made into other items rather than needing to be processed. Coin are in a sense a placeholder for value and it will take a strong player-made system that is more-or-less universally accepted by players to actually place value in coins. This includes trying to include those that do not have the means of creating a coin press or having any metal access at all into a coin based system.
And just because something is potentially infinite doesn't mean that it is worthless. Many people seem to use the curiosity, food, and other material as currency despite that they are infinite. A lot of it is based on the rarity of the item, the amount of effort it takes for the specific item to be made, and how fast it is removed from the system. For example, Straw dolls may not see too much value despite that they are one of the better curiosities because they are fairly easy to make. On the other hand, a peal is more valuable because it is used to make an item that gives additional LP learning rate and the fact that you only have a 1% chance per mussel boiled to get one.
A last thing of note is that I will leave this topic here tat seem to have been borderline discussing what we have been talking about here.
http://www.havenandhearth.com/forum/viewtopic.php?f=5&t=10837