Because legally it is not possible to actually leave the Euro, the only way to actually do it in practice is through the issue of a parallel currency. But Germany happens to be the only country which cannot exit the Euro in that manner, because of Gresham's law that "bad money drives out good". The New Mark would be the "good money" to the "bad" Euro, and so the New Mark would be hoarded while the Euro continues to circulate at a depreciating rate. This applies only to cash transactions, however. Electronic transactions would still be conducted using accounts denominated in New Mark.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman