Weapon Potency runes apply to spellcasting attack rolls and DCs.

Big boost to spellcasters, and brings their average chance of getting the enemy to fail/crit fail up from 40% to 50% for a big portion of their career. A bit powerful at the highest levels, with the +3 on top of Legendary, but honestly, when you're at level 19+, you can already rewrite reality. That, and I tweak up a few spellcaster enemy DCs to compensate at those levels, so they're equally scary, and it all works out.