The magnet plate is not intended to be a secondary diaphragm. It's intended to hold the magnet, which is necessary to the creation of a magnetic field, which is necessary to make the diaphragm react to the current running through the voice coil. Nothing is more hyped and yet simpler than a headphone driver. The potato chip bag (mylar) has a low mass, allowing it to be faster than the old paper cones. It's form fit to oscillate in reaction to changes in the electromagnet field produced by the magnet and the voice coil. The magnet is a cheap dynamic costing less than $1 retail. The voice coil is just high gauge (very thin) copper wire. The best voice coil is "flat wire" rolled together a few mm wide. The voice coil is run from the magnet and glued to the diaphragm. It's a fragile assembly but hardly complex. Its most sophisticated aspect is the shape of the mylar, which has built-in ridges that look like a shrimp plate. These ridges give the diaphragm borrowed rigidity for bass. The center of the diaphragm also a built-in ring to facilitate the employment of the voice coil.
With regards to the magnet plate, the only reason it's metal is to help it hold the magnet, which sticks to it without any glue. You can actually take a screwdriver and pop the cheap magnet out of the plate.
As for the bloated claims of the other top headphone manufacturers, ring magnets are not much more expensive. Grado could employ a ring magnet and completely do away with the back plate. It would take some retooling, however.
Grado employs a dab of dynatek-style damping on the magnet back, a dab big enough to damp the free ringing of the magnet back without covering up the two ventilation holes. I came up with my own cheap-and-dirty felt damping that doesn't cover the holes. It looks like this:
