Note for international readers: I'm British, and Britain is currently a member of the European Union, but is not part of the core group of EU members who have signed up to the single currency. Whether we should be, and whether we should be more or less part of the EU, is a big wrangle for us at the moment.
In summary: I think we want less rather than more EU. I confess that my principal reason is a slightly sentimental instinct that we British don't do very well with Grands Projets and we're better off muddling along with our own little way of doing things that seems to work. The French are good at big plans; we're better at improvising. But I have a number of more thought-out ideas, which give some rigour to this instinct.
The principal and most topical reason is that I'm against the single currency.
The advantage of having lots of different currencies is that different countries can have different economic policies depending on what's going on there at the time. In practice what this means is that a country that's growing slowly (in the long term this will be for cultural reasons - compare Germany and Spain, where cultures put different values on economic effort) can either (A) devalue, so it can export its uncompetitive goods or (B) cut interest rates so its uncompetitive industries can get funds more easily and its poor consumers can borrow to buy.
In the long run (A) and (B) come to the same thing: the country gets poorer SLOWLY. But it's the 'slowly' bit that's important. After all, economics is ALL about change: there is no end state, no equilibrium. What matters is, first, that the changes should be positive - well, everyone knows that; but second, which often gets missed out, that if you have to have changes for the worse, they should be SLOW changes. Because fast changes break up families, villages and cultures. You only have to look at South Wales to see this.
If South Wales had had its own currency it could have devalued slowly, letting its coal and steel remain marginally competitive whilst the worst performers went one by one to the wall. Instead everything shut down at once, and the social consequences are easy to see. Now, of course, I'm not arguing in favour of a separate currency for South Wales (in fact it wouldn't be a completely stupid idea - but the disadvantages outweigh the advantages - I'll come back to this in a moment). But the larger the scale of your single currency, the bigger the problems.
Everyone says 'look at the USA, it works fine there'. But the USA is a young country with a single language, very few entrenched local religious or cultural loyalties, and a unified conception of nationhood based on capitalism and the constitution. So when the economy of S Virginia goes down the tubes because coal becomes uncompetitive it's not a catastrophe, because a third of Virginians rent U-Haul trucks and go off to Texas and get jobs in the oil business (and after a while property gets cheaper in Virginia and new companies move in, and the remaining two thirds are ok as well).
That's fine, because they're all Americans, and although they're sad to leave, they fit in when they arrive, in much the same way as a young man from Swansea who goes for a job in Norwich feels homesick but not exiled. On the other hand if the same thing happens in Portugal, and a million Portuguese have to go off to Norway to find jobs, that's a horse of a different colour. Because either they simply won't go, and they'll be reduced to abject poverty; or they'll go, and two thousand years of culture, all the ties that bind, get chucked in the fire.
Now, the EU has seen this problem coming and has a solution: structural adjustment funds. But this solution is even worse than the problem. Because now, to be a succesful Portuguese politician you don't have to concentrate on running Portugal well, you have to concentrate on persuading Brussels that your region is going to hell in a handbasket so you deserve lots of centrally allocated Euros. And that way lies corruption: probably of individuals, certainly of political culture.
The main respects, then, in which it's better rather than worse for South Wales not to have its own currency are (A) that it's not a catastrophic disruption for people to move from Newport to Norwich to get work because (i) they speak the language (ii) there is a big cultural overlap between E Anglia and S Wales (iii) it's easy for them to keep in touch with family back home; and (B) because the authority charged with moving money from rich areas to poor areas to help regenerate S Wales and hold its life together is the British Government which, though far from infallible, has been in existence for several hundred years, is in the bottom quintile of corrupt institutions, and is reasonably close to the problem.
On the other hand, to sum up, a comparable readjustment inside a single European currency would result in either (A) bad cultural disruption as people uproot themselves en masse from their native sod; and (B) corruption, as structural adjustment funds are allocated through immature institutions.
There are a whole bunch of other reasons, too - let me do a quick brain-dump of the top contenders:
Er.... that's it for now. Is that enough? If not, tell me and I'll think up some more reasons.