Reverting to the Drachma will allow the Greeks to devalue their currency and devalue their debts. They will then also be able to set interest rates to suit their economic state and try and damp down boom/bust cycles. Here in the UK the independant Pound and central bank have achieved this with a reasonable degree of success.

The key has to be a controlled and managed transition, that can be explained to the electorate. The current austerity measures are politically unpopular, and probably unenforceable.

I happen to know that Drachmas are being printed and stock exchanges are testing their Drachma trading systems, so it really wouldn't come as a surprise to the investment and banking institutions.