PvE (sans raids) basically boils down to slaughtering mobs until you can advance to the next area where you slaughter more mobs, with the occasional object that you need to scan or boss fight here and there. You keep slaughtering mobs to find equipment that will let you slaughter tougher mobs. That's about it. Pets wouldn't really add a new dimension to the game. They'd only serve a primarily cosmetic purpose while perhaps improving an attribute like our damage output/defense/radar etc. Not terribly exciting. And you can forget seeing these things in PvP. Bungie can't even balance a shotgun properly. I don't want to imagine the nightmare that trying to balance pets would cause. If pets were to become a thing, they'd need more utility and things we can do with them aside from reducing them to arm candy and a stat boost. Companions should flesh out the experience. Right now, they would just be trivial.