[Radek]

AFAIK, the "-craft" skills control only one thing: they determine which spells you can gain - both during promotions and by using books of magic. Naturally, they also determine which spells you can cast from your spellbook because you must gain the spell first and only then you can cast it. Another thing that the skills seem to affect is the selection of books of magic offered to you in the Mages Guild and the Temple.
I haven't noticed any difference between casting a spell at "-craft" skill 7 and 14. The spell strength is controlled by the Sorcery skill.
Let us summarize. The "-craft" skills are rather "guards" than abilities. They prevent you from adding too strong spells to your spellbook too early. They force you to think a bit during character development. But they do not affect the spells cast by you.