Eschewing the Biblical parallels and such, I always figured Eru's motives -- and quite likely a lot of the motives of the Valar -- were beyond the full comprehension of the Men and Elves who recorded the tale of the Akallabeth. Manwe and Melkor always struck me as somewhat akin to the twin brothers Nissyen and Efnissyen of the Welsh Mabinogian, two halves of a whole in terms of good and evil (although Efnissyen is less absolute in his evil, since he repents in the end and sacrifices himself to destroy the Cauldron and save the Welsh forces from its armies of Undead, something I can't picture Melkor doing). It is said in the Silmarillion (IIRC) that Manwe cannot comprehend evil, having within him no evil. This is the first time that Valinor was directly assailed by armies of the Eruhini; my suspicion has long been that Manwe simply didn't know how to handle the situation. The Secondborn in particular have a fate unknown to him; any major action against them might subvert that destiny. So he falls back to asking Eru to take a hand, since He will know what the situation demands. It seems a rather extreme measure -- not unlike the Great Flood, since one has to assume there were innocent children in the world, even if every adult in existence was corrupt -- but by this point, it may have been the best solution to a problem the Valar themselves began when they did not deal with Melkor more expediently, and brought the Elves to Valinor, rather than trust that Eru would protect His Children while the Valar handled Melkor. Because he is good and without evil does not mean Manwe is free from mistakes. Perhaps if he had called upon Eru for help a long time before, the destruction of Numenor would not have happened. If the privilege of dwelling in Aman had not been granted to the Elves, they and Men would have had to find ways to coexist in Middle-earth -- with Melkor and his minions subdued before they had a chance to so thoroughly spoil the world.
Well, that's what's been burbling around my head for a while.
