The book "The Making of the Atomic Bomb" (Richard Rhodes) has a lot of interesting information, and different perspectives, on the atomic bombings. There's the traditional "we would have so many causalities" argument that's always made. But there were some other possible motivations that are very interesting, and the cynic in me tends to give them a lot of credence.
(Oh, and the book is fascinating, I think, a very good "geek" read. Things like the Nagasaki bomb having never been tested- there was so little Plutonium available they didn't want to waste any on a test, and they were *that* confident in the design they didn't feel a test was necessary. They just did the math.)
First, the development of "The Bomb" had been horrendously expensive, and mostly a "black" project. It was all going to come to light after the war. People involved in it needed a win- you couldn't have spent all that money for no reason and not expect to be crucified when it became known. So- "Bomb something, do it quick, we have to use it to justify having developed it!" Political CYA.
Also, there was a huge amount of concern about the Soviets. They had developed this huge army, and we didn't like how they thought- they were becoming the enemy. We needed to demonstrate that we had the ultimate big-stick so there wouldn't be any mistaking who the toughest kid on the block really was. Chest-thumping on a massive scale.
And also, there was concern about having to share the control of Japan after they surrendered. Things weren't going so well in Germany and we simply didn't want to have to include the Soviets in the process. If the war went on the Soviets involvement would necessitate including them... if we ended it quickly though, before they were really involved, we could leave them out. Like not sharing these subs
Personally I suspect all of these issues (and more) played into it. I think the "too many american lives would be lost" argument isn't really sufficient on it's own. But second-guessing secrets from 60-some years ago... who knows?