does a switch in an old work box require a ground?
Like Stubbie said, it isn't exactly code compliant, but in the practical world, it isn't the end of all life as we know it. We ground things that have exposed non-current carrying metal. What you don't want to do is actually connect the ground conductor to either end, because if there is a fault at one end or the other, then the ground would allow the voltage to leave the area and be available at the other end. And there are plastic plate screws available.
I would say that ideally you'd change the wiring in the circuit to grounded wiring. Barring that, it would be nice to have the circuit GFCI protected. And foregoing that, you could eliminate any exposed metal, such as the plate screws.