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?