Chaos magic. Is it necessary?

Well, you can definitely combine light magic effectively with enchanted arrows and a ballista. I think that if your guild has Divine Vengeance, light can even become more useful than chaos. And triple vengeance (if you have an enchanted ballista, of course) is perfect against some powerful attackers (like Cerberuses). If the druids help, of course. Also, if the enemy hero has switched to light, you can cast Remove Curse on the arrows, removing magic from several stacks (well, it depends on how lucky you are with sworn enemies) and, of course, use traditional mass spells to significantly strengthen your army.

P.S. I'm saying that you can also successfully use the more traditional light magic for elves; you don't necessarily have to go into chaos.