Astral doesn't blow stuff up. It's most "awesome" spells are cast in support of a conventional army, a force multiplier. And used like that it's incredibly powerful. A lot of its applications require you to mass spellcasters, but with them you can massively weaken the enemy army and strengthen your own.

One battlefield spell makes your entire army lucky and another curses the entire enemey army. This on its own turns an equal battle into a slaughter.

At a lower level spamming Paralyse breaks an enemy formation into bitesize pieces, reducing the impact of its attack and allowing yours to beat it piecemeal. Again, turning an equal battle into a winning one.

You can kill enemy SCs without having to build your own.

Enslaving a few key units can turn a battle. I once, as Arco, fought a battle with Caelum. Two enslaved Mammoths turned the tide by themselves.

Armies with many magical units become terribly vulnerable to being destroyed or taken over (again, with battlefield-wide spells).

And Communion gives you the flexibility to be able to cast the high-level high-fatigue spells without spending (and risking) a fortune in magic items.

Except in the very early game - when Raise Skeletos is King - Astral on the battlefield is much more powerful than Death (Drain Life is a fine spell, but only hurts one target, and needs a D4 caster). And this is OK, because Death has many excellent summons which Astral doesn't.

Agreed that Air is also very powerful, but in a different (rather more direct) way, and has different applications. I wouldn't want to cast Wrathful Skies in support of an army - my elephants wouldn't thank me for it.

I like that the magical schools are different, and have to be applied in quite different ways. It's what makes this game so interesting. Appealing for more summons for Astral just leads us down the path of homogenising the game in pursuit of fairness or balance, and losing both the flavour and the challenge of learning how to use all the different options currently available.