At the David Laidler Festschrift, John Crow, former Governor of the Bank of Canada, pointed out how the rising price of oil has put upward pressure on the Canadian dollar. He further pointed out that while the oil patch is primarily in the western provinces, the rising international price of the Canuck buck has reduced world demand for other Canadian goods and services, which has particularly inflicted some economic pain on firms in the eastern provinces.
What if there were two different currencies, one for eastern Canada and a different currency for western Canada? The western currency would certainly appreciate relative to the eastern currency, and this led Clark Leith (who was presenting a paper about currency unions) to reply to John Crow's comment,
Canada is not an optimal currency union.
At that point David Laidler interjected that labour mobility is important. Within a given country, differential shifts in demand induce considerable labour mobility, which tends to attenuate the shifts that would otherwise occur; if there were two different countries with two different currencies and with barriers to labour mobility, a currency union would have much more difficulty.
In that sense, it is easy to understand how crucial it was/is that barriers to labour mobility be reduced within the Euro community in order for the Euro to survive as a common currency across different countries.
Related: David Laidler also pointed out that the US treatment of Canadian
softwood lumber producers indicated that when a small country signs an agreement with a very large neighbour, it has little recourse if the very large neighbour abrogates the agreement. He used this example to argue against forming a currency union between Canada and the United States.