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.