Of course, this assumes the case that also other weapons hadn't been developed as well.
One of my favorite weapons, where my username is derived, was actually very effective in it's own right.
http://en.m.wikipedia.org/wiki/Bat_bomb
Of course, a bat bomb wasn't a sure thing, but Fat man was much more complex than little boy, which wasn't as efficient, and we didn't have much more nuclear material than a few bombs worth at the time.
So, in hindsight, I don't think it's easy to say whether or not the atom bomb was absolutely necessary to ultimately prevent many lives (on both sides) from being lost. I do believe it was very effective to that end, however, with the side effect of letting the nuclear genie out of the bottle.