[Lazy_Perfectionist]

Sandman said:

For sacred units which have low protection and no shield, the astral bless is better.

Really, how? Twist fate works once. Get hit by a slinger, and its gone.
As Atlantis' fastest unit has 9 AP, Twist Fate will never survive to close combat.

and magic resistance is always handy. Even better, astral nations can use spells like Body Ethereal, Luck and Anti-magic to further enhance the astral bless,

Very true

whilst the air shield becomes obsolete with Arrow Fend.

Partially true. If your sacreds are very expensive, like a 150 gold Niefelheim giant, you're going to give them proper support. If however, your sacreds are a cheap 25 gold, its nice to have taskforces you can support with only one commander and yet tear through independents or province defense while your real army engages the bulk of the enemy force.

If you take a peek at man, you need one Magister Arcane to provide the arrow fend (250 vs 130 of Tungalik), a Magister of Theology (40 gold) to bless and carry the spare gems. The compact taskforce obviously isn't a great idea here. You'd be investing 930 gold for an equivalent force of sixteen. Whereas I'm spending 530.

So, if you have an inexpensive commander who can bless and lead a relatively inexpensive squad of sacreds who aren't that powerful compared to other Elites, but like a Chainsaw compared to the boring old day to day troops... An air bless can give you a lot more mileage to your troops- though they're not great for the epic battles.

The air bless needs help. Ideally, it'd be a good bless for air nations like Vanheim and Caelum.

Actually, I agree. It would be nice if it wasn't as redundant. However, it is worth noting that unlike both fire and water, there are no shock-resist spells, and lightning always hits if there's something in the square where it falls.

If you look at Caelum- I could think of all sorts of wonderful uses for the Shock Resist bless if only they had flying sacreds. Unfortunately, the earthbound are at a measly 5 AP. I suppose you could use them as bodyguards to prevent melee messing up your script while like the last spell your mage casts is Shockwave. But with their current options and those Storm Guards there isn't anything that shock resist can do that you can't do sooner with non-sacreds.

On top of that, your sacred Earthbounds have Kite Shields and 19 protection, rendering missile warding rather useless. Most of your mages can also cast air shield or arrow fend as well. You'd only find a use for the missile protection if you were spamming LE Caretakers in some kind of bizarre strategy. (Earth Boots+Summon Earthpower -> Attack -> Point-blank Bladewind -> Retreat?). Anyways, air bless is a waste...

On Vanheim (EA), you've got rare air focused chassis like Asynja. And then you have Vanhere sacred infantry and sacred mounted commanders with air magic. Other than research, there's zero barrier to getting up Arrow Fend. Use two gems instead of one, and you're not even taken out by the fatigue. However, Arrow Fend does require Enchantment six, though Air Shield is available immediately.
If you take the air bless, you can support with a simple Vanherse of 160 gold and 8 fatigue per cast. If you rely on magic, then a Vanjarl will cast the spell for you, but falling unconscious in the process, and costing two gems and 280 gold. Alternatively, you could get the Vanadrott out for 380 gold, spend two gems, and take about 58 fatigue instead.

I really didn't find any valid rush strat using the air bless, though I'm not familiar with the race. I'd say it's also a bit crummy, though considering the high defense, it might be combinable with Shock Wave - barely.