What benefit of Bitcoin (to governments) outweighs the cost of not being able to control the value of a fiat currency?
Using a foreign currency as legal tender can curb inflation and delimit deficit spending. It will also reduce friction for trade with other countries accepting the same currency.

In some cases the stability provided by using a foreign currency outweighs the benefits of controlling your own currency. 
 [Wikipedia: Currency Substitution] lists for example
 ten countries that use the USD exclusively, as well as numerous other countries that do not maintain their own legal tender.