As a historian who taught courses on World War II, I think the exercise isn't entirely bad. I would have assigned my students to construct TWO arguments, one for dropping the bomb and one against dropping the bomb. I would have put the exercise in the larger context of "Moral Issues and War" in which we also would have addressed "Should the Eight Air Force have diverted B-17 missions to bomb the mechanisms feeding the German Work Camp System?" using the same comparative "pro and con" approach. That makes students think. Otherwise, you're propagandizing rather than educating.