Northern European banks are heavily invested in Greece. Kicking Greece out would require another round of bank bailouts.
Northern European banks are also heavily invested in Eastern Europe - which is experiencing a true depression.
Either way, is it really relevant? The imbalances caused by globalization with an easily expanded fiat monetary system is what's really the issue here.
You think Germany would have prospered all these years if the PIIGS didn't buy products from them thru deficit spending? The PIIGS actually LOST productivity when they entered EMU. The ECB, heavily influenced by France and Germany, kept rates extremely low in order for their higher paid workers to remain competitive with their southern lower paid counterparts.
These low rates created an environment in the PIIGS for the misallocation of capital, and subsequent bubble creation. In other words, their economies should have grown legitimately in the early 2000s thru increased production, but the Franco Prussians didn't want that - they preferred a system of dependent neomercantilism. So instead, these southern countries relied on deficit spending to create economic growth - which is the UK and USA business model. Now the Franco-Germans are paying the price.
Focusing on Greece is important beacuse of the domino theory danger. However, let's not forget the Macro picture - just as in 2007-2008 people used the expression "it's all subprime" one can argue today that "all western nations are PIIGS."