Although Greece accounts for less than 2% of EU GDP, a bankrupt state (or Grexit) could have serious repercussions.
1. Contagion. Italy, France, Spain, Portugal may need to implement (more) debt reducing measures. Risk premiums on for example Spanish bonds have already risen, pushing yields up sharply.
2. Confidence in the Euro as currency will take a knock.
3. The U.K. will realise that lender EU states will be obliged to provide huge rescue packages for Greece. Do they still want to be part of the EU ? (Nope).
4. Greece may forge much closer ties with Russia.
5. A crumbling Greek neighbour will help ISIS to destabilise the Balkan states - they are already at it.
So all in all, not a pretty picture.