We killed Rhulk sure, but how much do we know about Xivu Arath and her forces again? Hmm, the Cabal survived a direct invasion of their homeworld and have offered an alliance. Saladin gave his life because Crow took the life of a hugh ranking officer of the Cabal. Now he gets to work with the Cabal to strengthen them the way the Iron Banner has strengthened us AND he gets to learn more about them and how they operate. The Eliksni are with us too and right now are kinda free loading but we still benefit from them being here as well. They have their own resources to offer as well. Both of these factions were our enemies, now they offer peace in the face of our shared enemy. They know there is no point in fighting one another because that simply leaves us weak or dead. Information is a powerful tool, as is cooperation. The Cabal and Eliksni offer both, despite not being light bearers themselves. It would be foolish to simply turn them down and spend time and resources at war with them rather than preparing for what's coming.