Kershek said:
Well, skeletons still take half damage from piercing and slashing, so they don't need to be spell-soakers.
The point is that if you have some good HP'd skeletons, or enough of them, this can pose as a real problem since the ranged weapons aren't as effective. This is where blasting spells comes in.
This is especially true if the skeletons are merely a diversion to something posing a much bigger threat.