"Weak demons"??

I believe that demons can take Light magic, but they shouldn't. For several reasons. Firstly, there are few useful spells; Divine Force, Smite, and Anti-Magic are the only things needed here, while other spells are acquired through leveling up, and it is not worth sacrificing an entire perk for that. Secondly, these spells must be obtained from somewhere, which is a second problem: if you have only one Inferno castle, obtaining the spells is practically impossible (one should not rely on the small chances of getting a spell in the Dragon Utopia or the Temple). Personally, I believe Darkness is the only school truly useful for a demon.