Here's what Milton Friedman said about it in 1997: http://www.greekcrisis.net/2012/08/the-euro-monetary-unity-t...
The drive for the Euro has been motivated by politics not economics. The aim has been to link Germany and France so closely as to make a future European war impossible, and to set the stage for a federal United States of Europe. I believe that adoption of the Euro would have the opposite effect. It would exacerbate political tensions by converting divergent shocks that could have been readily accommodated by exchange rate changes into divisive political issues. Political unity can pave the way for monetary unity. Monetary unity imposed under unfavorable conditions will prove a barrier to the achievement of political unity.
If you are looking at it from a Brexit perspective, consider that shedding skeptic countries that never joined the Euro is a good thing in a federalist sense, since it makes it easier to achieve consensus in a smaller and more committed group.
In a way, losing Greece or Italy would have proven Friedman right, but the opposite happened: the Euro survived, Greece and Italy now are forced to drive their economy from a European perspective. De-facto political unity is happening, country by country.