Keep in mind, that when the decision was made to go for Iwo, the people making that decision didn't know about the Bomb. As far as they knew, Operation Olympic, the invasion of Japan was the plan. In that case, Iwo was critical to the logistics of the plan.

Also, as a B-29 emergency field... it sounds chitty, but at the time, Marines were cheaper than B-29 bombers and their crews.

Intellectual honesty is the most important character trait in human beings.