One of the largest benefits of national currencies is when they're healthy everyone uses it and knows the value making commerce 10000x easier. Look at the EU one of their major projects was creating a single currency to make things much simpler to facilitate trade between their members. Now imagine the US 'private currency' situation, it's like the European market pre-Euro but instead of the boundaries of the currencies happening at national borders it's just an undifferentiated mass.

The US had a mix of currencies at one point as local and state banks issued their own bills and it was a mess.