One more thought on the "one currency" issue. At least in the US, legal tender (monopoly) laws punish anything used that isn't dollar based. You are required to

One more thought on the "one currency" issue. At least in the US, legal tender (monopoly) laws punish anything used that isn't dollar based. You are required to accurately report capital gains you acquired from currency value fluctuations. This is an IRS/accounting nightmare for most people, hence multiple currencies are avoided by most people.