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This would work better if the switch worked as you described - close a circuit to GND to turn the card on, leave the circuit open to turn the card off.

If the wires to the switch fail then the card fails on.

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Not necessarily. An open wire doesn't mean 0 volts -- it is whatever the input of the gate floats to. There is no standard -- it depends if the manufacturer put a weak pull-up or pull-down into the chip, or if they have an active termination that will pull to either end. Or if the gate isn't designed any particular way, it could be subject to how well doped the transistors of that particular chip turned out. Being open also means it is subject to noise from adjacent signals, so it could be random that way, too.

It depends on what you're looking for. If you're worried about security you probably want it to fail off instead of on.

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