When money has value you run into a problem of whatever commodity you tie it to gets tied up as money. Using a renewable resource (and as little as possible) for this seems like the best solution to me. That way you can actually use gold, silver and copper in the applications they are useful for instead of storing them somewhere.
It also allows increasing the size of the economy as much as you want. If you tie it to gold you have to increase the value of gold as people are born or have massive mining for "printing" more money. The classical issues before moving away from the gold standard was running out of money to expand the economy since there was no more gold...
Currently there is an estimated 171,300 tonnes of gold. There is around 7 billion people, which would translate to 24 gram per person...http://i.ebayimg.com/00/s/MTYwMFgxNjAw/z/WXYAAOSwAH ZUNVds/_3.jpg
So then you run into coins with little gold in them (or whatever you pick out), which goes back to no value money.