I'm sure that PT will come back to you on this. My take is that the Euro is a composite currency. So if you add the value of the Drachma and the Mark together and then divide by two... you get a currency which is either a cheap Mark or an expensive Drachma. Cheap Mark = really good for German exports. Expensive Drachma= really bad for Greek exports.
In a very real way, the Greeks (and other club med) have been subsidizing the German economy.