Ah, I see I forgot to mention; Every specific use of magic is a skill. So you cannot just go "lightning Magic". Far to powerfull and broad a think, see? Magic is the skill you should have in that place, and then you have to distribute spells in place of skillslots. So that the more powerfull a mage, the less other skills you have. Balance as well as a representation as to how much time, effort and specialization it takes to master magic.

This is why Battle Mages tend to have one-two "tricks" they refined. Ie becoming a human flamethrower, or  conjuring lightning from their hands. Their focus is split, thus they lack proper mastery for complex stuff.