Quasqueton said:
I just think saying "double damage" is simpler than saying "-10 on saves, or if no save then double damage". Actually, the new way is less damaging; maybe the designers didn't want elemental spells/effects to do more damage. They just wanted them to do full damage. Maybe I've just answered my own question?
Basically it seems they don't want combat results to depend so heavily on a single die roll. An empowered fireball is often effectively an insta-kill spell when cast on cold creatures, if it does double damage on a failed save.