1. Damage does not scale. Your enemies health do.
2. Disproportionate mana cost/damage ratio for higher lvl destruction spells.
3. Spells are boring.
4. Perks does not add anything interesting to game play.
I'm so happy that I abandoned this school without sinking and perks into it.
Why conjuration is the bread and butter school.
1. Summons scale - your daedra lord will still be the most powerful creature outside of bosses as you lvl up.
2. Bound Weapons scale better than normal weapons and look cooler (bound weapon perk)
3. Awsome Perks - summon 2 things. Bound weapons auto soul tap, bound weapons banish summons are among the most useful mage perks.
4. Best Mastery lvl spells - being able to summon permenantly is insanely good.
5. Cool factor. Atronarchs look badass/having a daedra lord under your command?
6. Summons fill in other roles - frost atronarchs/daedra can tank an incredible amount of damage. Storm atronarchs have ranged chain lightning...except more damaging.
Kind of sad that conjuration is overwhelmingly better than destruction.

I use lots of different spells though from all the spell trees. Variety is the spice of life! Do find it pretty annoying getting 1 hit as a mage because I have 100 hp and no armor, probably need to use more alteration stuff.