There was indeed a political argument to throw the bomb (Look commies, we got bomba!) but Japan could not be allowed to just sign a peace treaty and carry on. They had too many warcrimes on their head, and even after the bombs were thrown, the Japanese got the Americans so far to not prosecute the emperor - Which pissed off an awful lot of Europeans.
As horrible as the use was, I really don't see how another year of fighting, bombing and suffering is anymore better then the two weakest atomic bombs we ever had.