[At this, Celestia turns to regard Elsa fully, an expression of frank surprise on her face.]

It is always better to make an enemy into a friend, and an ally out of a foe. If you expend your strength to destroy them, at worst you have weakened yourself to do so, and at best you have gained nothing. As troublesome as Discord is as an ally, it is better than having him emerge every time the Elements of Harmony change hooves, with another and better plan to overcome them.