This would actually be a genius move by a country like Cuba or Greece. They could bypass their external monetary issues, and they could maintain sovereignty by issuing their own currency through their own central bank backed by bitcoin (like what is done with gold and other reserve currencies).. by the very nature of them adopting the price would soar. Other countries like Venezuela, Iran, etc.. that has some monetary gripe (right or wrong) then would have an increasing incentive to jump on board. As the early adopter Cuba or Greece type country would become fabulously wealthy.. almost like if you were the first country to officially recognize gold as valuable.
Why wouldn't they just make their own cryptocurrency?
+1, they wouldn't but they would undoubtably f*ck it up.
That said, compared to Bitcoin, Cuba and Greece are extraordinarily wealthy economies. That might not always be so, but its true now. A) Greece is in the European union, at least for now, and I think there is Zero chance of the ECB letting Greece exit the euro for Bitcoin. B) Who would be mining on Cuba? The Cubans would have to buy all their bitcoin from us, and the second it came out Cuba would move to bitcoin, we would charge them a trillion dollars to get anyway. So switching would probably impoverish them.
Countries would either start their own, or just plain not switch. What you could see is a rise of the 'shadow economy' where all bitcoin users are members of a pseudo worldwide economy.