Quantum computers are only useful where you don't deliberately want decoherence. Decoherence means an operation when you erase a bit, like for example when you overwrite a memory bit with a new value. This requires dissipation of energy and interaction with the outside world to reject the heat of the calculation to. While in principle a quantum computer can do a calculation that a classical computer can do, it would not be useful unless it was observed and this happens pretty much every time a logic gate output flips in a classical computer.