Actually it's called immersion and suspension of disbelief. Explaining game mechanics through lore is what allows them to be interesting and fun. Bungie asked us to be engaged in a story driven game so justification of these mechanics hardly seems weak. Sonic doesn't ask you to do anything but run and the story is just a token addition to the game. As far as being bullet sponges that is just how shooters have been designed to keep action flowing. I don't like it but that is the nature of the beast. At least in Destiny we are zombie light constructs and arguably not even alive so the bullet sponge thing is a bit easier to swallow.