First of all - Jack of all trades aka mages with all schools is by any means a viable choice for this game. Although you still should combine - one school in 5, all others in 1 e.g. And I would recommend 3 schools (2 at choice and at least level 1 in witchcraft) anyway after some hundred hours of play.
Second - yes, you are missing. Quite something. You missed the ingame tooltip telling you that all spells cost (much) more action points if you do not have the required level of magic school. And you missed some quite important level 5 talents, like lightning rod or environmental immunity (air and geomancer). Last but not least - level 5 allows for an unlimited number of spells for the respective school. Which suddenly lets you detect and use some formerly neglected spells which turn out to be quite powerful in some fights.
Regards,
Thorsten
