Either the coil wire, or the cover, or even the baseplate can act as a microphonic element, because if they're free to jiggle around, they can cause a voltage in the coil that corresponds to the frequency of their movement. In order to prevent feedback, its good to have some wax covering all those surfaces. I don't believe there are any sonic benefits to having a microphonic pickup. I think that's a myth. Logic says that if the pickup is microphonic enough to pickup of the sound of the guitar strings, then you should be able to talk into the pickup as if it were an actual microphone, and with even a light amount of gain it should feed back like crazy, the same as if you cranked the gain on an actual microphone. I don't know why people believe a non potted pickup sounds better, but I don't see any rationale as to how it could be tonally beneficial.