One of the reasons Iwo was taken was to be an emergency landing field for the B-29’s that were bombing Japan. Before the island was secured, several of our planes made emergency landings there. The decision was made at the time with the conditions that existed at the time.
I've always wondered if that was true or another one of the rationalizations someone came up with after the fact. I do realize, however, that since very few people knew about the bomb the island was taken with a much longer and more costly air campaign in mind. The aircrew casualties they were expecting are another group of lives the bomb spared.
It was also an early warning outpost for raids and a base for fighters that attacked the inbound and outbound bombers. Could it have been bypassed? Probably, with the acceptance of greater Air Corps casualties. Would those casualties have equaled the Marine Corps and Navy casualties in taking the island? We can argue now until the cows come home, but remember in February 1945 the atomic bomb had still not been tested, and it was not known how long the bombing campaign would continue.