Awesome point, Iarwain. That's actually how I view God/dess, Spirit, the Guy in the Sky, or whatever you want to call that supreme being we all try and get to like us. And I believe it works here, too. The Song does come from Eru/Ilúvatar, and all parts or themes of it come from him/it as well. Therefore, I guess that Eru is neither good nor evil, he simply is everything. (Though this is not to say that he supports Morgoth in any way
) Good and evil need each other to survive, I think, because they play off and feed off of each other. No doubt Eru could destroy Morgoth or any other evil in a second, but he's wise enough to realize that this would throw off the balance that good and evil have, and this would basically wreak havoc on the world. (And as an afterthought, someone else evil would come along later anyway... how cyclic life is...)