[Aiwendil]

Tolkien's view of the relation between good and evil has always seemed rather Augustinian to me. In The City of God, Augustine says:

The sins of men and angels do nothing to impede the "great works of the Lord which accomplish His will." For He who by His providence and omnipotence distributes to every one his own portion, is able to make good use not only of the good, but also of the wicked. And thus making a good use of the wicked angel, who, in punishment of his first wicked volition, was doomed to an obduracy that prevents him now from willing any good, why should not God have permitted him to tempt the first man, who had been created upright, that is to say, with a good will?

For who will dare to believe or say that it was not in God's power to prevent both angels and men from sinning? But God preferred to leave this in their power, and thus to show both what evil could be wrought by their pride, and what good by His grace.

This strikes me as being remarkably similar to Iluvatar's words to Melkor:

And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.

The idea here, whether one thinks it wise or absurd, is clear: the existence of evil enriches good. I suspect that Augustine would say that the "grace" imparted by Jesus's sacrifice surpasses even the Edenic human state. In this connection, it's worth pointing out that what davem said here is not quite correct:

The elves look forward (if we can use such a phrase about such a 'backward-looking' people) to Arda Unmarred - Arda as it should have been sans Melkor's rebellion

It is not just Arda Unmarred, Arda Hastaina, to which the Elves look forward; it is Arda Healed, Arda Envinyanta. Tolkien makes the Augustinian point quite clearly through Manwe in "Finwe and Miriel":

The second is the Unmarred that shall be: that is, to speak according to Time in which they have their being, the Arda Healed, which shall be greater and more fair than the first, because of the Marring . . .

And again in the "Athrabeth", through Finrod:

For that Arda Healed shall not be Arda Unmarred, but a third thing and a greater, and yet the same.