This sums up what I think : http://www.youtube.com/watch?v=998tSBoa3lo
Nuking wasn't a necessity but a political move, as mainland Japan was easy to take over at that time : their naval fleet had been decimated, most of their elite pilots had been killed in kamikaze attacks, people were starving, 67 big cities had been litteraly destroyed by six months of nightly raids and the drop of millions of incendiary bombs (except for those which were kept untouched by Air Force so accurate assessment of the bombs could be made later on and Kyoto which would have been the first to be nuked but was saved by the fact Henry L. Stimpson, Secretary of war, had fond memories of his honeymoon in this city).
USA had already planed the invasion of Japan but the Russian had a head start, In one day fight they had recovered Manchouria and annexed the Kouril Islands and had plans to invade Hokkaido. Had the Russians entered Japan before the rendition they could have claimed a lot more than what they got. USA took this opportunity to impeed the Soviets to expand any further and at the same time scare them with the bomb and this was the begining of Cold War.
Of course the bombs ended the war quicker but was it really worth it ?
