Casting damage makes it hard for trainee/low ranked priests and mages to manage their meager resources.
It often leaves trainee characters frustratingly impotent - they can't help someone without killing themselves.
New members don't feel confident to ask for healing, or are afraid to waste party resources.
It is easy to forget about casting damage when you are trying to get the hang of all your new stats/abilities/vocals.
It can't be bandaged off, so it drains the trainee ranked healer's precious power after each fight.
By rank 30 it has become largely irrelevant, as skills/foci/talismans/life have been purchased, so it doesn't increase the challenge for higher ranked players.