[jpatokal]

To put it another way, if a government decides to issue some new, incompatible currency, and whatever currency you had previously lost some or all of its value, would you trust that the new currency would not also be replaced by yet another currency later on?

And that's not just a hypothetical, as it's precisely what happened with the Myanma (Burmese) kyat:

http://en.wikipedia.org/wiki/Myanma_kyat#Banknotes ("Third kyat" onwards)

Multiple times, in fact, eventually leading to this:

http://en.wikipedia.org/wiki/8888_Uprising