Leaders who are mildly depressed are needed in time of change because they see things differently than someone who is "sane". Mental illness allowed some of our greatest leaders in history to make actual social change by using radical empathy, realism, and the ability to adapt under constantly changing circumstances.
"When traditional approaches begin to fail, however, great crisis leaders see new opportunities. When the past no longer guides the future, they invent a new future. When old questions are unanswerable and new questions unrecognized, they create new solutions. They are realistic enough to see painful truths, and when calamity occurs, they can lift up the rest of us."