Again, and I repeat this AGAIN, in order for Greece's exports to become more competitive, Greece actually needs exports to begin with.
And going back to the drachma is not going to result in Greek labor being cheaper. Keep in mind this is SYRIZA in charge, a very pro-leftist party that isn't going to just do away with the regulations associated with labor protections. They have been very resistant to the Troika's demands to liberalize the labor market in Greece (and thus make it more competitive) and will only do so when forced.
As a matter in fact, Greece's biggest exports are refined petroleum products. But Greece's most important export, requires imported crude oil to produce, that crude oil is valued in dollars and as a result it would do almost nothing to make Greek refiners more competitive or profitable on the global market.
Tourism, another important industry for Greece, wouldn't save Greece either. While a lower valued drachma would give tourists more purchasing power, Greece isn't going to look very attractive to tourists when people are protesting outside because of unemployment, austerity, high costs of living, etc.
Devaluation is not some magical wand that fixes everything. If that were the case, every country would try and make sure that their currency was worthless. Even in countries that have the export economies capable of handling a lower currency can only go so low before it becomes dangerous, just as Wiemar Germany.
Except Greece is ****ed outside of the EU. I have already said this so many times, outside of the EU, Greece will see lower volumes of trade because it will be restoring barriers between it and its largest trading partners. Prices will go up due to fewer goods entering Greece, increased costs of trade due to currency exchanges, possible tariffs, a currency that will be garbage, etc. Greek interest rates will be far too high. Greek savings accounts would be wiped out. Greek corporations and banks would still have debts that would be valued in euros, which would then become harder to pay off. And without some kind of deal about Greece's debt, Greece is going to have a hard time finding new lenders, with or without the euro.
Also, switching over to a new currency will be expensive. Computers will have to be reprogrammed, payment and vending machines will have to be re-serviced, printing presses will have to be made, and capital controls will have to be put in place in order to ensure that people will get the needed drachmas.
But of course, I am very willing to bet that you will literally ignore every logical point I have just made and you will continue to act as if Greece will be all fine and dandy once they leave the Eurozone.