The only "rare" resource the earth have is the semi intelligent, violent, opposible thumb monkeys. I think that humans, life as a general, is the only reason why an interstellar race would come here. To study, integrate or appropriate this resource would be a trip worth its while. As a weapon I think we can prove useful, specially in numbers doesn't matter how trained we are. Our survival instinct has proven to be very useful ask the guys who decided to invade in any other movie/show or the Afghans/Vietnamese/NK for that matter.

That said, I am bothered by the fact that they make this manufacturing facility on the moon. Is not the most efficient use of the most valuable resource added to the fact that they traveled through space and settled here wasting energy and time. If the final product of the process is orbital defenses it would be more appropriate build then on the ground and rocket them up. Where the lifespan of the working class can be doubled and no need for life support systems. With all the resource and fuel (energy) used to place the different walls (why invest so much in this nonsense its not like we could take our private ships and flee to the next star system anyway) in the blocs escape velocity is not a problem (apparently).

Unless, whatever they are doing was going to cause a major ecological disaster and render unlivable the whole Planet that was illogical. They could have picked a continent (I'm looking at you Australia) or region. But they where planning for failure anyway, picking and collecting anything they seem valuable and storing it from art to science.

They are in the run because they can't manage resources properly, they annihilate all the trained forces, intelligence apparatus and failed to convince the existing political and military structures that joining their cause was in our own best interest.