They are probably saving it to add later or in a sequel. There aren't a ton of multiplayer PvP ideas out there that work, so I'm sure they are going to trickle all the good types out over time. Makes sense. That's how games stay going over long periods. Look at GTA5. They were supposed to have Heists from day one online and 11 months later, they still don't but say its on the way. As long as Bungie doesn't sit on things as long as Rockstar, it should work out well.