Get an onyx medal more than once

From experience, a single command post is sufficient to occupy the money and man-power of two players. Unless you want to spread out or, as Tom-grochowicz mentions in his answer, build a back-up base, there isn't a lot of gain to be had from building this in the early game.

However, if you are playing with more than two people then the additional defences unlocked by having a second command post can be used to compliment each other. For example, you might want a turret per person, which you will need at least another command post for.

When you haven't been playing Horde mode long, there are certain unlocks for Horde 2.0 that a second command post can help with:
The Infinite Ammo mutator for Horde requires you to Earn a Combat Engineer ribbon 100 times. To earn this ribbon, you must work on five fortifications in a single round [source]. A second (or more) command post will provide a new set of potentially cheap barriers to build and repair, which will enable you (and others) to get this medal faster. This will also help to level up your fortifications faster.

When you have been playing horde mode for a while, you will have achieved some or all of the fortification levels and unlocks. Many of these make fortifications cheaper to repair and maintain, which means you will find yourself with more money as you progress. You may find that you simply have a cash excess that you would like to sink into more barriers and decoys. For example, with barriers, repair becomes cheaper at each even level:

• Level 2 - Spike Barrier Repair Cost 8% Less - Level 1 Decoy Unlocked
• Level 4 - Razor Wire Repair Cost 8% Less - Level 1 Turret Unlocked.
• Level 6 - Electrified Barrier Repair Cost 8% Less Level 8 - Laser
• Barrier Repair Cost 8% Less

The reason for the weird system popups is that the Xbox doesn't support 3D natively. It's just the game itself that outputs two half-size images on a normal 2D output. The TV then pulls out the two images and displays one to each eye when you put it manually in 3D side by side mode.

If you look at the Xbox guide during Gears with the TV in 2D mode what you see is what the TV uses to split the two images. It will take the left half of the screen with the left half of the dashboard and display it to the left eye (only) in 3D mode. The right half of the dashboard goes to the right eye (only) so the two eyes see totally different content.

If the dashboard knew that the game was in 3D side by side mode it could potentially show two smaller squashed and repeated dashboards, one on the left and one on the right so that both eyes see the same image in 3D, but it doesn't.

Also you're not seeing a 2160x1920 image. You're seeing a 1280x720 image as generated by Gears, with each half of the image dedicated to one eye. The 360 then upscales to a single 1920x1080 image. The TV further upscales the image for each eye to fill the screen. So it's 640x720 per eye upscaled to 1920x1080 per eye on an active 3D display, or sidescaled to 1920x560 on a passive 3D display. Either way it will look blocky.