Sure it's reasonable. I think bitcoins have zero value. I recognize that there may be people out there who are willing to pay real dollars for it right now, but I think that in the long run, no such people will exist. Hence, I place its value at zero.
Bitcoins don't have to be tradable to dollars to represent nonzero value. Their utility is in and of itself valuable. It might not have value to you, but that's different than suggesting that it has no value at all.
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.
