I just had a crazy thought: What if there's an enchantment coming that grants a creature the Intercept trait?!
Suddenly White Cloak Knight can protect his entire zone (e.g. your mage or whatever) from non-Zone attack spells. Perhaps that's why he's worded as he is. That is, he could have been worded that he "can't be the target of attack spells" - but instead, they went with making him immune to the damage and effects, allowing him to still be the target. Normally these would be equivalent, since you wouldn't choose to target him if there is another legal target... but what if there isn't another legal target in his zone because he's on Guard and has Intercept?!?!
And aside from the obvious combo with White Cloak Knight, an Intercept enchantment would be a pretty strong counter to Wizard's Tower, along with other ranged attack creatures and conjurations - even Ballista would become less powerful if I could give any (living? corporeal?) creature the Intercept trait.
Perhaps it will be another Nature enchantment like "Cat Reflexes" or "Gecko Hands" or "Eye of the Tiger" or "Spidey Senses."
Or maybe a War enchantment (targeting any Corporeal creature): "Bravery", "Kungfu skills", "Grabby Fingers."
(I imagine it would probably in an Arena expansion, though, as Intercept isn't as useful in Academy)