Well, when the light came out of Kingdom Hearts - arguably, the most powerful source of light there could be by definition - it vaporized Xehanort's heartless (a being of pure darkness) but left Sora and company unharmed. So I don't know how any light so pure or powerful could be used as a weapon against the good guys.
-_- Ah. Good point. But what about in some scenes when there is a bright light, Sora covers his eyes? Like right before the first battle with Xemnas at Memory Skyscraper. Xemnas thrusted his arms up and light shot up from the ground and then grew very bright. Sora covered his eyes, didn't he? Then again...Riku and he walked into the Door to Light without covering their eyes. .....-_- If I were in this game, I'd bring sunglasses with me.