By the way, offensive magic is still less effective than support magic.

I can't help but agree. No matter how much you level up all these lightning bolts, ice arrows, and fireballs, you still constantly feel the urge to use some sort of time stop, petrification, or mind control. Even a simple speed boost for your creatures can sometimes yield incredible results, compared to which offensive magic is completely overrated. But while air/water provide additional stunning/freezing effects, fire is completely useless.

The most useful light spells: dispel magic, aura of retribution III, and the light elemental (undoubtedly the most dangerous of the elementals).

I haven't used dispel magic even once. There hasn't even been a need for it. Retribution is also not very good compared to other schools (unless we are masochists who lose troops left and right). As for the elemental, I also can't agree. All elementals, in terms of their physical strength and considering the small number of them that can be summoned, are quite weak. I prefer earth elementals. They are very convenient to throw into the thick of enemy troops to distract them. Considering their resilience and regeneration, this creates a good tactical advantage (especially if you use them to cover your ranged units). Plus, they can cast petrification. It works devastatingly well against the AI. And since I usually level up earth magic for various healing and petrification spells, summoning elementals also becomes quite effective.
In general, I believe that light magic is useful exclusively for healing and blinding, and is not good for anything else. Although, it depends on who, what, and how they play ^_^