The Orion Nebula

I am curious, are these buildings totally useless?

Assuming I am Uni, and don't care about morale, why should I bother even building these? If someone is going to invade my planet it's pretty much a given that they are going to bring enough transports to run it over, or they will just bomb from orbit. Production would be better spent on actual active defenses like Missiles and Fighter Garrisons, and then you are wasting all that BC per turn maintaining them.

The way I see it is you only need a barracks on a few colonies to make your own transports.

Am I missing something?

Granted, but a marine barracks costs 60, each transport costs 100, and combined with an armor barracks can make your opponent spend thousands of production points (not to mention command points) building and maintaining (and transporting) transports to the appropriate planets.

Of course if you're playing against AI you probably don't need to worry too much about multi-planet invasions, so in that case I wouldn't build either of them, unless it was for the purpose of raising morale.

I am curious, are these buildings totally useless?

Assuming I am Uni, and don't care about morale, why should I bother even building these? If someone is going to invade my planet it's pretty much a given that they are going to bring enough transports to run it over, or they will just bomb from orbit. Production would be better spent on actual active defenses like Missiles and Fighter Garrisons, and then you are wasting all that BC per turn maintaining them.

The way I see it is you only need a barracks on a few colonies to make your own transports.

Am I missing something?

Sometimes you need to build many transports fast, to start your own invasion or to get back planet taken by the enemy. You can't do it with only 1 planet building transports, at the same time losing several turns waiting till they are built could be fatal. Consequently when time comes you need to have enough marine barracks to be able to build transports in 1 or 2 turns.
As for armor barracks, in original moo they are never taken...besides they can't build transports.

Those kinds of defenses become more important, if for your Large and Huge planets near an opponent or you are Warlord (or plan to pick it later).

The maximum number of troops and armor you can have on a planet is effected by the planet size. So, If you have a 42 max pop planet (Aqua,Subt+maxed Terf'd+BioSp+AdvPlan), and you are Warlord, then, building both of these makes perfect sense.
Your opponent will have a much more difficult time taking your planet, and if they fail, your Warlord race produces more back at a faster rate, so, it becomes even more important to get it right, when attacking this kind of foe.

By the same token, it would basically be a waste to build these on Tiny pop 3 planets.

Personally, I don't research Armor Barracks, because, I prefer the other tech more, that I would have to give up, to get it.

Is maximum number of units on the planet affected by biosphere?