If you are playing as Magnar or a faction with "Flesh bound Tome," and choose Raise Skeleton Horde as a mage, you get a huge source of free xp. Use Cull the weak on the skeletons and you'll easily recoup the casting cost, and pocket some serious mana (20x9 = 180 mana - 42 casting cost = 138 free mana!)
Given, it does take work to seek out easy battles and constantly summon then cull skeletons, but using this method Magnar can raise insane mana and with magic research be casting Curgen's Volacno very early game.
Not a MAJOR ISSUE, but making Cull The Weak not work on the undead makes sense to me.
