A couple points here that people tend to forget. The Xterra "weighs" 4000 lbs. That's when you lift it vertically. When you are pulling it, you have tires to roll on, and it weighs only a fraction.

The factory tow hook can easily handle the weight of the vehicle, and it's only attached to one side of the frame.

The reason the corner pieces were not retained, is due to the damage you can do to your body. Look underneath the plastic corners sometime. There is painted sheet metal body there, that goes all the way to the bottom. My sheet metal is bent on both sides from obstacle departure dragging.
I would much rather have 3/16" steel down there than plastic.

Steve did mention to me that the bumper will probably be 3 pieces, made up of the cross section and the corners, to aid in shipping.

As for replacing a bumper, that's asthetic. A bumper like this, or an ARB is designed to handle damage, and shouldn't be "replaced" if you back into a tree and dent it.

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Regards,
Ian