Logically speaking, our Guardian has literally swathed through hordes of enemies, gunfire and any possible name for a sharp object swung their way. While we can just have armor and all rematerialized by our Ghost, you would be able to survive and keep fighting until your next rez so long as you had armor left to take the damage even if it got blown off or stripped away from gunfire, bit by bit. Perfect example: Cayde. As tough as an Exo’s body is, those Scorn still managed to throw him against that steel grate/panel hard enough that his face got smashed up. I don’t care where you’re from, that had got to hurt. That said, had he a visor or some sorta protector on to take the physical shock from that, since his horn wouldn’t provide much in a helm fitting, he probably would’ve fared better. It just makes sense that the Guardian would have a helm on in combat zones seeing how they’ve seen the most combat out of most and thus would use past experience to account for it all.