[HerenIstarion]

I believe that in the case (as well as elswhere, and even if unconsiously so), Tolkien was employing the Christian idea of humiliation as the means of reducing pride and making one capable of receivibng heaven's gifts. To put it short - without utterly shattered hope, the Eldar would not have been ready to accept help from the West (the point being, we are so good at beating Morgoth, who cares what that Mandos told us back than, ha!). The loss helped tham see that there were no other resources to rely upon but the West for them, while the victory would simply postpone the defeat