It's a simple question really. We, as Guardians, hear many stories about how the Eliksni home-world was destroyed by the Whirlwind (like ours) and how they were left by the Great Machine (likely the Traveler), but they are our enemies somehow. Why would we not unite under the light and fight as allies? We could keep Earth (with some Fallen colonies) and the Fallen could take Venus (with some human colonies). We could help give them a home and reclaim our own. Who knows, if it worked we could possibly help retake their home-world. Plus, it would be awesome to have Eliksni Guardians on our side of the fight.