Ah, I see, the usual problem/con that much more power is lost to national/local governments over distant federalist/global ones.
The pros are certainly the convenience of traveling country to country in Europe and not having to exchange currency to engage in commerce.
But yeah, there's a whole pro/con to a global currency idea, I don't think I'm for it for the most part, but it's something I'd have to really examine more.
When I went to Greece a few times I got burned because they switched to EURO and before that Americans could have a good/easy time given the exchange rate and bringing USD to buy/exchange for the former Greek Drachma... Your buying power during a vacation was extended, but then it went the other way with the EURO, and so they lost tourism/business at some level.