The argument was made, for instance, by Wolfgang Münchau: Euro and trade will make EU irrelevant (FT.com, January 27, 2013)

What is the point of the EU beyond the eurozone? This is not nearly as flippant a question as it may seem at first sight. It is, in effect, the question Britain will ultimately answer if it has a referendum on membership. But since the preservation of the eurozone is turning into the EU's principal preoccupation, what is in it for a non-member such as the UK?

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As a loud critic of the eurozone's crisis response, I am under no illusion about the speed of a secessionist single market. The current agreement for a banking union does not qualify. But the eurozone will ultimately end up with a proper union that covers all banks, and which includes resolution powers, deposit insurance - and usurps whatever is left of the single market for financial services.

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Faced with the combined development of a eurozone economic union and a transatlantic free-trade zone, the added benefit of EU membership loses appeal if most of that benefit can be had outside. If one is absolutely certain that one will never join the eurozone, there really is not much of a point to being a member of the EU.

In the long run, we're all misquoted — not Keynes