While I remember reading (somewhere) that the main point of having as much in the main book as possible is to cut down on the Sourcebook Syndrome that most other RPG companies indulge in, I have to say that questions like this one are definitely a reason why sourcebooks are not necessarily a bad thing.  I mean, yeah... it's great to say that foci will be a system in the book, but what about players who want more than that?  I'm perfectly willing to take the book and make my own rules regarding artificing or potion-making or anything else, but I may be the exception to the rule.  And there will always be people who will want to have "official" rules, no matter what.

For me personally, if there isn't something written in the book, then I'll make up my own rules and try them out.  If they work, great and I'd let people know about them.  If not, then no big loss.  I tried, right?  But maybe a sourcebook on magic wouldn't be such a bad idea.  Just to add some width and breadth to the topic.