The concept of nations is a relic though. We're not under threat of roaming Hun tribes or whatnot. The exclusive modern purpose of a nation seems to be to have a currency, and the purpose of the nationally administered currency seems to be to maintain the nation... Seems terribly circular to me.
If we admit that geographically defined nations are silly then I don't see any reason why we can't just let people, businesses, or perhaps even entire cities, choose which currency they wish to subscribe to.
"The exclusive modern purpose of a nation seems to be to have a currency"
That is incorrect. The purpose of a nation is manifold, nations provide a lot more than just a currency for example: rule of law, public services (roads, fire service), judicial system, safety regulations, an open and efficient marketplace. Currency is a means to some of these ends not a purpose in and of itself. Now like everything else in life not all of these tasks are always well executed and thats why governments do things like change laws and policies and provide for mechanisms such as the courts and a free press. I believe you are taking a very narrow view of what a nation is and what it (strives) to provide.
They do when it matters. Many countries at various times have used the dollar as a non official but widespread currency, eg much of south america, and Russia.
If we admit that geographically defined nations are silly then I don't see any reason why we can't just let people, businesses, or perhaps even entire cities, choose which currency they wish to subscribe to.