A valid point, but how many rotors and sprockets do you know of that have been balanced?
I doubt the inertial moment of a guard that diameter and weight would create any perceived vibration issues....much less at the speeds a dune/trail bike normally sees.

A skid plate is simpler in construction and does the same basic thing (perhaps a bit heavier?) except for a loss in ground clearance and preventing chain derailment like a sprocket guard can.

Now, for the sprocket guard, instead of having it rotate with the sprocket, why not a similar approach as the rotor guard?
If you had it extend over the sprocket covering the chain, from the point at which the chain meshes with the sprocket to the point it departs, then you'd have what amounts to a case saver for your sprocket.
The chain wouldn't have a chance to derail, the sprocket/chain would be protected, and no additional rotational mass or the unlikely vibration issue.
Have to admit, the blingstar sprocket guard looks pretty sweet, but it looks heavy as ****.
Maybe some cutouts or lightening holes would be a good idea.