For any remotely plausible definition of "worthless," bitcoins are just as worthless as any representative money, which inclues fiat money (like a USD) and commodity-backed money (like a gold certificate).
The US government is a powerful entity that levies taxes and accepts tax payments only in dollars. Gold has various uses in industry and fashion and therefore a certificate entitling one to ownership of gold would have some value independent of gold's use as a currency. None of these are true for bitcoin.