in the affairs of men and elves is a tricky business, from any point of view. It's like a teen-aged sister or brother intervening in a dispute between toddlers: you don't have their point of view, you don't have a vested interest in the outcome, and your intervention may just make the whole business worse! Sometimes, you just have to let the little ones fight it out among themselves, and learn their own lessons. As I recall, the elves learned to reject the blandishments of the dark forces, but men and dwarves were still having a problem with it, chiefly because of their respective greed for power and riches. I think the sending of the Wizards to assist in the fight was probably the best course the Valar could take: it guaranteed the balance of power while leaving everyone free to make their own choices and learn their own lessons.