Just like Thaumcraft, spells here takes advantage of main hand weapon.
Vanilla or modded
Enchantments or preset
It doesn't matter

From sharpness, looting and else
To armour piercing, "permanent damage"
Etc

Even "Backstabbing" from farmer's delight mod
Doing it job right, making some kind of roguish spellcasting, where behind spell hit gain damage modifier (Based on enchantment level)

I think...

It could be adapted
Or even balanced
Like... main hand weapon may take damage for using it as catalyst

This feature could be removed somehow...
What if spells here are "two handed"?
(+Player cast animations could be nice part of it)

Or... well
It could be forgotten
Because this already works fine
Little bit Brocken, but fine

P.S.
Or maybe this is actual feature?..