I'd like to add that being useful isn't a negative for something's properties as a currency - for example, the reason cigarettes become a currency in a lot of jail/P.O.W. situations is because they are always liquid due to smokers needing to smoke.

Often in those situations, cigarettes come into the economy as a ration (or a purchase limit) that people get whether or not they are smokers, and the economy revolves around getting this commodity that everyone gets to the minority that need it. Pretty quickly, you end up with non-smokers trading cigarettes with each other, because you can always get whatever you want through paying a smoker in cigarettes.

Bitcoin could benefit from that if it encouraged a very large drug market, because people need their drugs. If an easy, comfortable bitcoin drug market could get a lot of volume, it would become a lot easier to buy a cup of coffee or a car with it, and the prices would rise and fall based on the relationship of the ease of purchasing drugs with bitcoin vs. the ease of purchasing drugs in dollars. Maybe money laundering could keep bitcoin stable, too. In any case, to be successful, IMO bitcoin will have to be driven by something other than belief in bitcoin.

The use of the USD? It can be used to pay US taxes.