It's quite fascinating, really. First of all, excellent analysis. On the flip side of things, however, consider how the masked solders are typically the bad guys. Look at the classic Stormtroopers from Star Wars, always wearing helmets, versus the open-faced variants that the rebels wore. This motif of a masked menace is epitomized by Darth Vader himself. And in other fiction as well, such as - more recently - the Hydra soldiers from Captain America, who all also wore concealing helmets. Typically, the good guys' faces are visible to the audience exactly for the reason that Caveman offered: so that we can relate to them and more easily identify them as the heroes. It's really a testament to Bungie's storytelling and character development that they managed to draw us in so tightly with the ever-masked Master Chief and the rest of his nearly inhuman SPARTAN allies.