I must say that's an unusual way to do spell damage in my experience. I've always thought that if you cast a spell which damages multiple creatures, like
cone of cold,
fireball,
lightning bolt, etc., you should roll the damage dice once. This in effect says that you've managed to weave a specific amount of magical energy into that particular casting, with the only modifiers to damage dealt being on the defensive end (saving throw success or failure, abilities like evasion, resistance, immunity, etc.). Plus of course when you're blasting a large number of foes, the sheer number of dice that might need to be rolled just gets silly.

Your thoughts?