I don't really define it in such harsh terms as being completely invalid or fool's gold, just that because Sybil protection doesn't exist, it's security through obscurity, where the only way anyone actually knows the security of the system at any given time is for you to know the total hash rate and acquire 51% of it yourself.  I think there's a distinction to be made between provably secure, provably bad or invalid security, or in the case of Bitcoin, an unknown level of security to most or all parties at all times.

As Smooth said, such a system can still have value.  You don't have to be a perfect system, just better or competitive with the others.  Not hard to do when your competition is a Federal Reserve enslavement scheme.  It's like asking a prisoner would you rather be tortured with a chainsaw or be given a cell phone with bad reception.