But for SP? Does my party really have to be all Sourcerers? Why not pick somebody up along the way that may be sympathetic to my cause? Or another that's not so sympathetic, but good at hiding his distaste and following his own agenda? There are practically limitless possibilities here.
I suppose it doesn't HAVE to be that way, but Larian wants to make use out of their new Source Point system and source skills, and those only are useful for Sourcerers. The main characters also have a story goal to get to Black Ring territory, and non-Sourcerers wouldn't share that goal.
I can't really see any good reason for someone who is not a Sourcerer and hates Sourcerers to be hanging out with a party of Sourcerers who are heading for the territory of the Cult of Mustache-Twirling Evilness (you must be this evil to apply).
