Originally Posted by
Qualia
That's how it works. You only need to believe that you can wield the Light in order to do it.
Plus, while he never bothered with the Light in the past (partly due to the NE had little to do with the Light and partly due to Fel being the more destructive power so far), Illidan was shown a vision by Xe'ra. In that vision, he was wielding the Light wrecking havocs to demons and evil of the Voids, that his followers / fellow soldiers looked at him in awe and wonder, and he looked "calm, strong and at peace". Seeing how much Illidan want power and to become something grander than his brother, I wouldn't be surprised if he is very willingly to hug and kiss the Light now (although he was still a bit skeptical about it in the book).