Any sniff of Spain leaving the euro then I and everyone else holding euros in Spanish banks will immediately transfer their money to accounts in foreign banks. It would cause the biggest bank run ever and cause a complete collapse of the financial system. The only way to prevent this would be for the Spanish government to impose capital flow controls overnight – a lockdown while everybody’s account gets converted into pesetas. I think this is quite unlikely so I really think Spain, or any of the economically weaker countries simply cannot leave the euro for this practical reason.
So if anybody is going to leave it will be Germany. I can’t see that happening in the near future, but you never know. Should that happen then it would be followed by the next strongest economy leaving as well, and so on until only the weakest economies use the euro, which by that time would be the euro by name only.
(BTW Those who think the political will is too strong for the euro to collapse should read up on Nathan Rothschild and understand that political will is just a drop in the ocean compared to the strength of the bond markets.)