The scroll for the Spoilspell rune states that it can cancel magickal effects or provide magickal protection. Is the magickal protection any good? What does it protect against?

Stuff like Firebolt, Energy bolt, etc. It's pretty handy. Pop a Power 3 Meta Rune, Ball Meta Rune, and Duration Meta Rune on it, and keep it up on your party in the later levels.

Also there's a wall later in the game you can dispel with it, in the Tower of Shadows.