My theory of the Exo is that they are indeed aliens. Many people seem to assume they are/were built by humans, but for all we know the Exo could be a race of aliens that transcended biological life, becoming 100% machine. This makes your title question difficult to answer, because we do not really know what the Exo are exactly. If they are cyborgs, than theoretically they can die, because death is the permanent end of all biological function in a living organism, even if that organism has enhanced itself to be primarily or almost completely cybernetic. Now if my theory stands true, and the Exo are sentient machines, then scientifically speaking they can not die, because they are technically not a living organism. Does this make them expendable? That depends on your definition of life. Technically speaking, a machine is expendable because it can be rebuilt and repaired. But what if this machine can think and act on its own? What if it can experience regret, satisfaction, gratefulness, vengeance and values it's continued existence, just as all biological life does? I don't see why it shouldn't qualify as "living". It's a very hard question to answer, and I probably went on more of a tangent then necessary, but as a final answer I would say no. The Exo are not expendable.

Bump

Does a copier die if you beat the shit out of it with a baseball bat?

Exo's never die!

I'm wondering if your characters ever 'die' canonically in the first place.

I always figured they were mostly cybernetic. That is, not just people with augmented prosthetics but human brains and bodies needing mechanized parts to keep the "meat" going. Maybe they've had to take up work as guardian mercenaries to pay off their debts on their parts; the lucky ones, who could afford the latest, kept themselves going long enough to pay off their debts and are now free, old men and women (what's left of them, anyway). Or... Maybe they were the injured from an old war that couldn't all be healed, and were mercifully or cruelly placed into suspended animation until some time when they could be? Now they've been brought back after the world's been rebuilt and the technology and means have been reclaimed, and most take up arms because it's all they know to do in an unfamiliar world? Either way, I feel their combat experience would make them too valuable to be expendable assets, even if they might themselves have a death wish. I think the Hive only need brains connected to matter in some way, not a prerequisite amount of muscle, bodily organs and tissue, but who knows? Maybe the Exo are used as tools against them and can't be infected? Besides, if robotics were so advanced as to fully replace the bodies [i]and[/i] minds of human soldiers, human Guardians themselves wouldn't need to go out and fight at all, don't you think?