The point is the cost-efficiency.
Because (totally imho) the strong bless costs a lot of points and... well... quite... s*cks.

75% SR? Not safe enough to toss around them Thunderstrikes and similar freely (not that you would anyway, you have far better and easier options); good to defend against... Caelum? Some others? Better than high MR and twist fate?
A strong bless is already expensive for the nice ones (F, W). I don't get the points of spending many points for A9, even because as vfb pointed out you just need and A5 caster for Dark Skies. You can have a mage cast it. Why to spend so many design points on A9? For which spells do you need it? S9 gets you Master Enslave on turn1 on battlefield, Arcane Nexus and a very strong Strands of Arcane Power for globals, and Wish as ritual
If you really want to be have an easy Dark Skies and protect your sacreds from arrows go for S9A5 i.e. but I don't really see many advantages over S9N4/6.
Also, I don't get how can you start searching for air sites "quickly" with an A9 pretender. At least you need it dormant, and anyway far less expensive indie mages can do a fair job in searching for those too.
I really liked your analysis of the A bless advantages (as you can see, I thanked you for that too) but if I really want some Air Shield just a minor bless seems enough to me.
