It is almost like life, spirit, and nature are one big spell class, similar to the elemental classes. Except where the elemental classes focus on damage and only damage, spirt/nature/life do all sorts of things... healing, damaging, buffing, regens, lifetaps, syphons, and so forth. I admit, I am no expert in the nature/life catergories, but I do think that those three classes are a bit too muddled together.
While their effects aren't quite as varied, the arcanes schools arent' strictly limited to damage. They have debuffs [Lessen Power, Lessen Focus, Stupidity, Scatterbrain, Stinging Cold, Sear, Freeze, Warped Perception], Roots [Ice Shackles], Super Unbreakable Root [Binding Crystals], Mezzes [Numbing Haze], Snares [Daunting Mist, Debase], Dots [Immolate], and the generic buffs [Ward/Resists]. Additionally some arcane spells can be teched with interesting effects [Stun, Burning Damage, Ice Snare], unlike the mystic line which just gets more generic versions of damage and range and minus % resistance. Basically...Arcane spells do all sorts of neat stuff too.