Originally Posted by
M-Ra
Problem is, he is important and valuable because he went that deep into the arts of fel magic and fights the Legion with their own Methods. Illidan as a Demon Hunter is a valuable ressource against the burning Legion. Turning him into some kind of Light Messiah would take away the one thing that makes him important and valuable. He is unable to control large forces, he is a crappy political leader and feeding his ego and his believe that he is some kind of chosen messiah is the worst thing you could do if you act with him.
Light Angel Illidan would be useless. If they want a Champion of the Light, pretty much every Paladin, even the weakest, would be a better choice, because they are more experienced with the use of Light Magic and in their case it wouldn't take away some effective weapon to fight fire with fire.