it mostly hinders against imps, which drain all mana, as the hero casts a skin spell or something else unnecessary, against harpies (in general, against all high-initiative creatures) it's also bad to play, as they manage to move and if you stand opposite them, you have to restart the game)) in my opinion, it's better to give a neutral bonus (+ to movement, or something like that)

Added after 3 minutes
playing as Dungeon, it's more beneficial for me to cast a poison spell or a vampire spell than a random spell.