Currency is a placeholder for value. So long as I can exchange bitcoins for value, they will function as a currency. The design of bitcoin makes it very difficult for governments to make such a scenario impossible (or even difficult). For that reason alone, bitcoins will remain valuable, if for nothing other than goods or services that governments don't like.

People who participate in illegal trade (such as drugs, guns, prostitution, etc.) still need most of their purchases to be in the legal economy. Hence, illegal trade still uses real currency.