The way I see it, it was neccessary and the easiest way to end the war. Japan would not have been an easy victory, it would have been a bloodbath that went on for years. They were the first non-European country to defeat a large European power in war. They were no joke.
Think about the Soviets for a second. Would it have been better to let them march into Japan? We would have had another Germany, Korea, Vietnam where we have to divide the land in half only to fight over it later (imagine a Japanese equivalent to the Vietnam War). The Japanese people would have been worse off, and the recovery of their country could have been slowed by decades. The Japan we know today would not exist, not even close.
They weren't exactly the nicest people during the war either. The Nazis are seen as monster (they were), but hardly anyone mentions the bullshit that Japan was responsible for during that war. They were as bad as the Nazis and in some cases even worse. Hirohito and his people got off easy, the whole country got off pretty easy if you ask me.
The Western Capitalist powers needed a strong base in Asia, and they needed to show that their system could work in Asia. I think it worked out in the end.
