When casting a spell you don't do any white damage. So discipline changes along those lines wouldn't be useful... which is fine in my books... a stat type doesn't have to be useful to everyone. Having said that, I've heard that spellcasters get panicky more quickly than melee BECAUSE they don't deal white damage... so discipline could still be useful.
If you do feel we need to cater more for spellcasters, maybe just make sure something like the chance of spell failure when in combat is impacted by discipline?