However, not knowing what an "Exo-shield" is (as opposed to a non-Exo shield), I could posit this rationale: Harvesters with them harvest fewer resources because the manpower is diverted to maintenance of the shield.
That's the general idea I was going for with the harvesters exoshields, yes. They are intrinsically tied to JUST a particular resource harvester, and they provide much better protection for it for the cost, so an ongoing "maintenance cost" seemed appropriate here.
Revenantus, I can also see why you'd want to make the two consistent, and part of me agrees with that, but in thinking about implementation I think this is one that would add more confusion and complexity (not the good kind) than it would solve. Usually I try to be as consistent as possible, but exo-shields and just general force fields are kind of two different things. If I implement per-space-dock exo-shields in the future, I'll probably make those similarly cheap and good, but with some sort of cost related to the dock (increased build times, or inability for engineers to assist building, etc).
This situation isn't entirely unprecedented - if the AI is gaining ground then your resource production and potentially manufacturing capacity will fall.
It's just another factor affecting how likely you are to be 'steam-rolled' in the event of a catastrophic mistake/oversight.
Good point. I don't think this will make a super-huge difference until the very last few minutes of a defeat, which by that point is probably predetermined, anyway. But, if you do survive at the last bastion, then getting another planet or two back becomes an even bigger priority. Since the AI does not re-colonize, that's probably a recoverable situation....