And people thought a flashlight was a useless weapon! Mahdi will definitely want to do his best to keep everyone's spirits up in time of crisis. He will probably exhaust himself using his abilities to lift people's moods and give blessings to people who are going to fight.

Mahdi also has the ability to become completely incorporeal (he basically transfers himself into thought), so he could potentially go to the surface to work as recon, or look for missing people. Unfortunately in order to actually interact with the world physically or even communicate (except potentially with someone who was a telepath or the like), he'd need to return to his regular, physically weak form.