Well soldiers need to shave to maintain grooming standards, no? Not that the gas masks aren't good enough reason alone...

I would think maintenance is probably the biggest determining factor in the decision between a DE and a straight razor. The handles are solid and can stand a bit of abuse. The razors are cheap and disposable, so you can just toss them afterwards. Also, gear would be lighter since you wouldn't need to pack a strop... or a honemeister.