by your argument, you can't know how dangerous a gun is until you shoot someone with it, or the fact that a back hoe could kill you without getting into an accident.
even if you don't know the power of a nuke, setting one off let's you know. like the Tsar for example(in addition to being able to calculate the power). Modern nukes are pretty terrifying. Americans have nukes with multiple warheads, each warhead is orders of magnitude more powerful. each warhead is probably powerful enough to wipe off the map entirely.
was it worth it? well, Operation downfall was going to be truly terrifying. literally millions of casualties on both sides were estimated, so ending the war sooner was better in terms of total life lost, but the losses were entirely civilian. mind you, you could also argue that dropping the bomb prevented us from using nuclear energy for every day use, as it now terrifies people. well... nuclear power is pretty good actually! would solve our energy crisis. not the environmental issues, we would just have different ones.
but I think the clearest benefit of the development of nuclear weapons is the lack of conflict between major powers. a conventional war between america and russia would have been brutal, and nuclear weapons prevented that. but I don't think that's the same thing as 'hiroshima and nagasaki'.
to summarize accurately: the bomb was not worth it, because of the unneccssary loss of civilian lives, but it cannot be accurately stated, because operation downfall (the final invasion of japan) would have cost millions of lives. it's impossible to know what would happen from there. it is entirely possible that they could have dropped the bomb on some forest, or Mt Fuji, and that may have equally ended the war.
