With the recent change to defensive structures, soldier outposts have become obsolete once you have researched the mass shield structure.

Soldier outpost
Size 2
Invasion defense 300

Mass shield
Size 2
Defense 450

Since invasion defense=defense/attack soldier outposts are no longer really a viable option to defending a planet as they are less efficient in terms of defense provided per space.

Higher level players can build a council base for invasion defense, but these are limited to one per planet.

Please either increase the defense of the soldier outpost to a more reasonable amount, or decrease the size to 1.

But it's still free, and it is quite useful when you don't have the money.

It's not free. While the cost of credits is minimal, political favor is a finite resource.

I see the problem being that invasion defense is a limited form of defense; you might as well put defense itself, and with the new changes the regular defense buildings are quite powerful. If a building is going to be invasion defense only, it should either be a higher value for the space, or invasion defense should have additional benefits (which I don't see happening). The problem is how do you equate regular buildings to ones that require political favor ? It allows you to potentially bypass your current tech level, but you might have reached that anyway by the time you unlock the building. It's clearly not supposed to be an end game tech.

The way I see it, they're meant to give you a leg-up. It's a fairly early tech unlock, and you can get political favor with an early-ish mission. Think of them as a defense bridge before your shield tech is upgraded enough to replace them.