So far as our having killed hundreds of thousands in a nuclear fire is concerned, yes in hindsight we probably didn't need to bomb Japan with these weapons. We now know that Japan would likely have surrendered after Russia entered the conflict. We know that we had killed equal numbers of civilians with conventional weapons and so this was really no different.
However, using that same hindsight we can also see that had we not used these horrible weapons on people, on cities and shown the world of common men what the weapons were capable of, one country or another would have used them at a later date. There was no true demonstration of the horror of nuclear weapons than actualy using them. All of the demonstrations of how destructive these weapons were - all of the pictures of mushroom clouds and houses being blown in, dummies being dismembered before they were burned to a crisp would never have had the PR power that a true bomb set off on an actual city had. Everything else would have been completely theoretical - not even the bikini Atol burst would have had the effect that pictures of swaths of destruction of a city had on the people of the planet.
The next war would have been fought without the understanding of the consequences. It could be said that those who died in Japan saved millions.
My father was an interpreter for the Italian navy during the Bikini test (and a number of others). They used this and other tests as measures of the theoretical. "what if this had happened dead center of San Francisco" and they extrapolated from there. The point was though that it was all theoretical. my father said that awesome (and the word actually had meaning then) as that burst was, it didn't have a real effect on Navy personel. Imagining the deaths of hundreds of thousands is not the same as an actual occurance.
I once asked him if maybe we should have simply done a demonstration for Japan. He said that although he believe that a single above ground burst should be made every 10 years for all the world's leaders to see, he didnt' believe that they could make that imaginary leap into their own cities.
Just a thought.