I think it is better if we don't do that since it'll plain expose half the cast's identities with complete obviousness. Not too concerned about giving an edge to caster because it isn't. Everyone knows that the 3 knight classes have the magical resistance class skill and that Saber tends to have the highest at norm. And unless they're Joan of Arc or something, even with an A rank magical resistance, Caster is using stuff beyond mere greater thaumaturgy and high ranking magic. Where a modern magus would be useless, try standing still and Caster could probably turn you into swiss cheese.
Didn't really know about the homunculus thing but I stand corrected? Is Luca supposed to be a vessel? If he isn't, you can prettymuch ignore, then.