They would need to be bonded because they are conductive, likely to become energized, and not grounded. You could likely have 120 volts wrapping around the face of the box and back behind the drywall unprotected. Shock hazard, arc fault, etc... In fact, if someone put a metal coverplate on an isolated ground duplex in this installation, the plate itself could easily become energized at 120volts and kill someone.
On a metal jem box, the hangers are electrically continuous with the metal jem box which is bonded to the equipment grounding conductor. So when a hot wire contacts them, the fault current travels through the madison hangers, then the box, then the equipment grounding conductor back to the bonded panel, turning off the breaker hopefully before any damage is done.
That being said, I very seriously doubt they are listed for use with plastic boxes just for the above reason.
Marks Brothers Electrical Contractors LLC.