Having options in all the different spell schools is a good thing. One thing to keep in mind is that the schools are designed for all the champions classes, not just mages, so you get some spells that are great for more martial champions getting up close and perosnal but that a back line mage wouldn't use. Regardless, each school has lots going for it:
Water is great. Slow is a great debuff, blizzard one-shot murders entire armies, mantle of oceans makes worrying about mana a thing of the past, and freeze gives you time to maneuver your armies into position on the strategic map.
Earth is light on damage spells, but has by far the best buffs in the game, especially if you take the geomancy and abjuration traits as well. Diamond skin and giant form makes ordinary troops unstoppable.
Air excels at mobility and control. Haste alone wins battles. Cloud walk is the ultimate strategic mobility spell and thunderstike is great for tactical mobility. Titan's Breath is the best crowd control spell in the game. Tornado breaks up big stacks into more manageable sizes. One of my favourite schools.
Life is the ultimate support line between your heals, growth and shrink. That dragon bugging you? Shrink it down to size and breath easy. It makes surviving battles easy.
Death is the ultimate debuff school and makes it easy for your troops to murder their foes with a few nice damage spells mixed in like horrific wail or kill.
Fire is probably my least favourite school. although it's pretty nice if you're going to bother going up the evoker tree (I rarely do unless I'm playing Magnar). Fire is probably the weakest school early game but puts out outrageous numbers late game if specced for full damage.