I feel like an idiot for not noticing that.
I read once that after a few turns, about half the opponent's creatures would be frozen at any time with Ice Shield/Permafrost Shield, so with Permafrost, this works kind of like a 1.5 damage reduction shield that prevents freezing when played. In that respect, I'd probably call the card less useful with Permafrost around; better to keep the creature frozen than play this card.