It is thematic. VERY thematic. It also hurts Jotunheim as they need as much gold as they can get and Niefelheim does not have any cheap patrollers. Niefelheim is very strongly biased towards bless strategy, that needs points. If you were going to cast Illwinter with your pretender you would have to sacrifice something. Giants kill everything they hit, period. +2 strength has next to none effect. And you probably wouldn't want to go above B5 needed for the spell. Water is nice, especially at 9, but that is very expensive again. You could also go for Earth or Nature, but not for both, and not for a nine.
If you were to cast Illwinter with your Skrattis you would have to wait until Contruction 6. W2B3 (optimal) would still need atleast Water Bracelet, Brazen Vessel and Blood Athame. You can get both Water and Blood boosters at Constr. 2/4, but you cannot equip them all as two of the Blood (Armor of Souls/Twisting Thorns) and the only Water (Robe of the Sea) booster all go for Body slot.
It is exceptional, but still not easy to use. And, as said before, you are everyone's target. It is not as Illwinter was the end of the world. It takes three years of Illwinter to make that happen!
Endoperez
That Last one is only a nordic myth, luckily