What about if you have a ground fault in your conduit?
With a EGC properly installed, as long as one end of my conduit is grounded properly, you have a safe, low impedance return path for the fault. If I'm using the conduit only as my EGC, then you need to make sure both ends of the conduit are grounded properly.
I know all 4sq boxes are listed so the concentrics can carry fault currents abouve 250V. Not sure about the Sq D disco you have, but if memory serves me correct, they too, are listed. However, it's easy to install the ground bushing rather than get into it with the inspector. Some battles just ain't worth picking, IMHO. And for the extra $2, I'll add the bushing to the list of materials.
Now 10 years ago, I would have picked this battle and argued until I won. I guess age is mellowing me a bit.