Yes, some entangled states could be useless for quantum cryptography. Quantum cryptography relies on the fact that entangled particles cannot be copied or eavesdropped on without disturbing their state, which would be detected by the legitimate parties. However, certain types of entangled states are known to be vulnerable to eavesdropping attacks.

For example, Werner states, which are a class of mixed entangled states, have been shown to be vulnerable to a type of attack known as the "entanglement swapping attack." In this attack, the eavesdropper can create their entangled particles and swap them with one of the particles in the original entangled state, without being detected by the legitimate parties. This effectively allows the eavesdropper to eavesdrop on the communication without disturbing the entangled state.

Therefore, not all entangled states are suitable for quantum cryptography, and it is important to carefully select entangled states that are resistant to eavesdropping attacks.