I kind of realized how important it is to us that we find SIVA first on Nessus, or at least contain it. Sure the Fallen are bad, but could you imagine the Vex getting ahold of that stuff and replicating it? They could probably perfect, and maybe even enhance it, further and faster than the Fallen ever could. Sure, all of the Vex we have met (besides Panoptes) have been mining models so far, but could you imagine a normal Goblin enhanced with SIVA? The millions of Goblins enhanced with SIVA? It would be the end. Making an already unstoppable force more powerful would be devastating, not even counting the Darkness coming as well.