There is a parallel universe in which the Euro was never invented, and the Eurozone countries kept their own moneys and their own central banks, with flexible exchange rates. I think the economic outcomes would have been better. But let's just suppose they weren't.
Suppose that each of those independent central banks had screwed up their monetary policies in their own independent ways, and we ended up with exactly the same pattern of unemployment rates and inflation (deflation) rates across the Eurozone countries that we actually observe today. The Bank of Greece in particular screwed up very badly in my parallel universe, by running an extremely tight monetary policy. I know this is very hard to imagine, but bear with me.
Would international relations be better or worse in my parallel universe? Would the Greeks be violating Basil Fawlty's first rule when negotiating with the Germans?
I think international relations would be better. I think that countries would be blaming themselves and not their neighbours. Flexible exchange rates would make it more likely that countries would take responsibility for their own predicaments, and start doing the reforms to monetary policy, fiscal policy, and economic policy in general, that they would need to undertake.
Optimal Currency Areas isn't just about asymmetric shocks, transactions costs, labour mobility, and fiscal transfers. It's about international relations and countries getting their acts together. Flexible exchange rate fences good neighbours make.
[I'm stealing Simon Wren-Lewis' thought experiment, and modifying it.]