Considering humans and their tribal mentality it will never happen.
Bunch of pretty silly answers.
One of the key achievements of 20th century economics was monetarism - they notion that the economy can be controlled by a central authority who can change the availability and cost of money. A good recent example of the problem with not having enough different currencies is the economic troubles in Greece. Prior to Greece joining EMU and adopting the Euro, in a similar economic situation Greece would have devalued the Drachma making Greek products cheaper and tourism in Greece cheaper. We would all be checking out the Parthenon in $22/night hotel rooms while stimulating the Greek economy. But EMU says they can't do that because the Germans hate inflation and aren't going to let it happen for Greece's benefit.
There are nice benefits to explicitly common currencies but in the end you can get most of them by using things like risk hedging in derivatives markets. Giving up your own autonomous monetary policy almost always seems like a bad idea to me.
BTW - The gold bugs will tell you that there should be a common currency (gold) which was the de facto common currency for most of the history of the world.
Is it really smart to have one anything control the whole world?
See the Euro for reasons why
-Because it hasn't gotten "That Far"- yet, in it's Maturity. And I'm afraid it will STILL be quite a-while- before it Does. :(
yes
ah
and why don;t we have just one language and country too?
