I'll have to look that book up. It sounds interesting.
My own theory more Western-astrological: the soul is drawn to a set of traits - not a name or income level but 'soul' or 'personality' or 'karmic' traits (Very very frequently (far more than statistical) there are obvious strong aspects between parents and children in their charts.) The soul is drawn to a situation that will put it into a position to learn what it needs to in each round.
In the case of Bill/Clark - Clark's life was cut short so he couldn't finish whatever lesson he was working on. Bill was born to parents very similar (personality-wise) to Jonathan and Martha and Bill is now working on similar lessons and he's going to do it without Lois.
