It seems unlikely that the trait is similar to the vampire trait, because otherwise it would be probably called “spellcaster”, besides the fact that there was no special mention to such a trait prior to the release of the vampires GP.
I may be wrong, but, from what I remember, the gurus described the trait as favoring your sim to become a spellcaster later in life, which is logically incompatible with choosing the life state right off CAS.
It must be something else. Maybe it makes the spellcaster more powerful, gives him/her an edge in duels or a larger energy pool to draw power from. It may also just be another path for becoming a spellcaster, but with special interactions or animations.