Monetary policy, used responsibly, is useful to manage public finances, provided it is not abused to issue too much currency. Greece cannot itself use monetary policy whilst it is part of the eurozone and adopting bitcoin would put Greece in exactly the same position.
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Monetary policy is something that should never be put in the hands of people. They will use it to enhance their own power, nothing more. It cannot be "used responsibly".
Deflating the currency hurts the poor, and helps the powerful. It is a tool to obtain and retain power.
Consequently the authoritarians in power in Greece (or elsewhere) would never go for it.