[ap22213]

I would add that currency needs to be convenient, as well.

There's no way that I'm going to lug around a block of gold, a scale, and snips when I have a credit card.

[gozur88]

Currencies can be redeemable. Until 1968 if you held US dollars you could take them to the treasury and trade them for silver coins (or bullion, post 1934). That gives you the benefits of precious metals without the hassle.