I've read a bit about this, about cherry picking the best spells. What I've found is almost all spells are situational to some point. What may be the biggest damage dealer may not always be what is needed. Since we also have elementals, a big fireball spell now heals fire based enemies. Another reason not to go big is space. The single hitters have all been comparable, so then you take the best situational one at the time if you have those. Then there are buffs/debuffs/CC's which all have a place. Not sure this is as big an issue as talked about.
A mage with 3 spells out of ervery single element will be way mor versatile than any mage that just focuses on 1 or 2 element with no real drawback. This just does not feel right.
1 rank each in fire. water, wind, earth and witchcraft = 5 ability points for 15 spell slots. Just for 5 points, thats almost free when compared what is required to get a single element to level 5 (which does not even give more effective spell slots unless there are more that 15 available spells for a given element).
