No one can even begin to discuss this issue intelligently without first taking account of the following facts: all the cities (Warsaw, Rotterdam, London, Conventry and so many others) that Germany and Japan bombed first, the fact that every day the war went on the Axis powers were slaughtering vast numbers of people, the facts of the Holocaust (which was killing thousands of Jews and other targeted peoples every day the war went on), the total "scorched earth" of the Eastern Front in which the Germans annihilated countless cities, towns, and villages, etc. etc.
The Allied Powers were trying to bring the earliest possible end to an aggressive war launched by the Axis powers which was killing TENS OF MILLIONS (final tally was probably 40+ million). Sure, one can argue that bombing of cities is never justified, but if it's EVER justified it certainly was in WWII as the best attempt to defeat the Axis.
btw, there's no certainty at all that the USSR would have stayed in the war without the US and UK attacking Germany all-out from the air, since before June 1944 that was the ONLY serious way they had to reduce the German military power on the Eastern Front. If Stalin had judged that the US and UK were unwilling to do all in their power to defeat Germany then he might well have made a negotiated peace and Nazi rule would have continued all over Europe, the Holocaust would have been carried out to its "final solution" etc.
Good post. You should read "The Conquerors" by Michael Beschloss. Great inside infomation on how Roosevelt and Truman were thinking all through WWII.