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