The reason I think Brandon Sanderson has taken this approach is to show the desperation of the light. After all how more desperate can you get if your champion goes mad.

The world and the forces of the Light are in bad shape. At this point, boys and girls, the Shadow is winning. There are glimmers of hope, but only glimmers, and they MUST pay off for the Light to win. All the Shadow needs for victory is for matters to keep on as they have been going thus far and one or two of those glimmers to fade or be extinguished. The forces of the Light are on the ropes, and they don’t even know everything the Dark One has up his sleeve.

Think of it this way. The bell is about to ring for the fifteenth round, and the Light is so far behind on points the only way to win is a knockout. Our boy is game, but he’s wobbly on his legs and bleeding from cuts over his eyes. Now he has three minutes to pull out his best stuff and deliver the punch of his life. The Dark One has taken a few shots, but nothing that has really damaged him. He’s still dancing on his toes and talking trash. His head shots can fracture a skull, and his body punches can break ribs. And now he’s ready to unveil his surprises. You didn’t think all it would take is for Rand to show up at the Last Battle, did you? According to the Prophecies, the Light has no chance without him, but his presence doesn’t ensure victory, just that the Light has a chance. Gotta stiffen your legs and blink the blood out of your eyes. Gotta suck it up and find that punch. Three minutes to go, and you gotta find that knockout. That’s your only chance.