To be honest, I have had mixed feelings about that optional rule. It makes characters emphasize casting power over casting skill, which I am not sure I like. It makes physical direct combat spells all but useless because of the high drain, but it helps pull Stunbolt back into line a bit. Since we don't have anyone set up to really abuse things, (I.e. 20 casting dice on Combat spells), I can drop that rule if you guys would prefer.
Remember, it affects your enemies too...