[The Squatter of Amon Rûdh]

I don't think that the defeat of Sauron alone was the mission of the Istari. I suspect that bringing the peoples of Middle Earth closer to one another seems a likely secondary objective, quite aside from the ethical points already considered in this discussion. Moreover, the all-out warfare on Melkor was a last resort, and only came at the request of Earendil on behalf of all the peoples, after a long struggle.

On a more realistic note, if the Istari simply destroyed Sauron themselves, there would have been nothing to tie this book in with The Hobbit (no need to find the ring, simply destroy its creator) and I don't think that the story would have been as personal or evocative. The way things were done just makes for a better story.