As others have pointed out, it happens in certain situations. Even worse, governmnets are willing rely on currencies issued by other governments in those cases: Not only do they not hold the pwoer over the currency as with bitcoin, there's even someone else holding that power.

Here's a few examples:
Montenegro and Kosovo used the German Mark and now use the Euro.
Many small states use the Euro.
Lichtenstein uses the Swiss Franc.
East Timor and Ecuador use the USD.

With the exception of really tiny states this usually happens in times of turmoil, civil war and major economic crisis. We could use that knowledge to make Bitcoin more widespread and to create a suitable global environment for further acceptance of Bitcoin.

Philipp