OK, I don't see any point in rolling mage specialized in some specific skill (like fire or air, or even fire+earth). It's seems to me that it is much better to spend ability points in all skills evenly. For example, it takes 15 ability points to level up geomancer to level 5 and get 11 earth spell as a result, but it takes just 5 ability points to level up all 5 magic skills to level one and get 15 spells as a result (3 spells for each school) or 15 to get 25 (5 spells for each school).
The only thing you sacrifice is a talent which has level 5 in a specific skills as a requirement (for example Demon) but they all look pretty weak to me. If you have 2 mages in a party, this approach works even better since you can split diffrent spells of the same school between them.
Am I missing something?