Air 9 blessing makes for a truly AWESOME combination used together with Wrathful skies - and is the best protection against an enemy using that spell if you have many sacred troops.
Additionally, it is a very, very, nice combination if you are fighting air-heavy nations like Caelum or Vanaheim.
It may not be as useful in as many situations as the fire and water blessings, but that certainly does not mean that it is weak.
And that is discounting the exceedingly strong power of the L4 blessing at high levels of air magic. For instance I love air 8-10 for Marignon (a Virtue is best for this) since it will allow my crossbows to fire straight into the same enemy formation my Knights of the Chalice are engaging in melee - and I don't need my Virtue along to cast "Arrow Fend" to get the effect, any priest will do.
A high level of air magic useles?? I think not.