Fire and water are notably inferior to the other six.
Nature has a full house of useful buffs/debuffs AND a good selection of summons, as well as a handful of spells that mimic the abilities of other schools (eagle eyes, protection, panic).
It's one chassis that has sufficient HP and holy 3. (Holy 4 when prophetized.) Thus it can take the hits, especially with regeneration... Not that Holy Avenger still is worthwhile usually.