Greece cannot leave the EU in its current financial state. The only thing propping up its disastrous economy is the euro. This is only because of Germany's leniency on Greece's financial difficulty. If Greece were to leave the EU, it would become immediately in debt to the rest of the EU, and would be auctioned off for scrap.
The Greek nation needs to gain financial and domestic strength before thinking about independence from the European Union. There is a problem with trying to stand on your own when your body is weak, the same goes for a nation. Greece has had so many domestic and financial issues that complete self reliance would be disastrous.
If Greece, or any country for that matter, leaves the European Union, then it would be a bad thing since other countries may follow suit. Greece needs the EU too much in order to keep its economy afloat. If Greece leaves, then Italy, Spain and Portugal may join the Greeks and then lesser economies might not join the EU.
I do not believe Greece should leave the European Union. I think the EU is a good way to bring together the countries within Europe under a single entity. I think this process works the best when the majority of countries are involved. I think countries should be concerned with signing up the remaining countries, not losing current countries.
Greece should not leave the European Union because they are part of Europe and that is the name of the organization. They can if they want to, but in my opinion they should not because the European Union makes more money than any other organization outside of a few corporations.