i have been involved in numerous should we shouldn't we have debates in several military history classes. i have heard no good argument for not using the bomb. the only valid argument in my mind is that we shouldn't have used the second one in Nagasaki.
the bombings up to that point had had a very pronounced effect on limiting the production capability of japan, but hadnt succeeded in convincing tojo and hirohito that they should surrender. the entire war saw the japanese completely detached from reality. yamamoto was really the only one that could see the empire crumbling, but he served his emperor honorably to the very end. they still beleived that they could win somehow, and couldnt see that they were defeated. the US was providing pressure on a lot of different fronts, and the bombs were just one small aspect of it. by showing what we were capable of, and them not knowing we had expended our supply of bombs, it provided a lot of leverage.
in today's world i would never advocate using nukes. for any reason whatsoever. i think it would be a better world if everyone dismantled every nuke on the planet. but when you think of how minimal the percentage of casualties for the war inflicted by the bombs actually was, and how much of an impact it had. war is not pretty, people die. but i think that by using the bombs it saved significant numbers of lives in the long run.