1. static emplacement are designated to be hardcountered by arty -off map or on map
2. i dont see any diference between air strike (single bombing pass) and off map arty barrage (for example railway one) - if you see at least one hen tell me it (except diferent off map [stuka is 1 skill shot , railway are 3 shots ] or in visual - russian 150kg bomb come with plane)
(3.)Only problem that is with stuka dive bomb is that it come often with recon overfly and in good doctrines.
(4.)Also i never seen it useful agains something else than static emplacements , immobilised tanks when i was playing agains skilled oponent
(I added points 3/4 in your quote, just so you know what I'm replying to)
1. I agree. cost vs risk makes it a no brainer though, especially in combination with 3. Howi is supposed to hard counter ele. Yet the good Ele doctrine comes with recon and stuka DB. Nothing in this game should really come without a counter, even if it's a light counter vs off map bombing run. The only thing that should, should be off map arty strikes, which generally are very expensive, and very RnG dependant (also see 2.).
2. There isn't really much difference, except feasible reality. If Relic designed a counter to offmap arty strike, they would be laughed at. It's just not a realistic outcome to be expected to shoot an arty round out of the sky. Generally this is offset, with the cost of said arty strikes.
(3.) see 1., as these are related.
(4.) I guess this is subjective. I've seen guys predict where a high value unit will move to, and put the strike there, or wait until a certain player is in a micro heavy engagement, then put the the strike in on another of his units (usually Katy's or such, that are away form his micro heavy engagement). Admittedly, this takes a certain skill instead of just click, it's still very cost efficient and potent however (particularly in CAS). I don't think it's unreasonable to give the guy in the micro heavy engagement the chance to light counter this, in case of mistake, with an AA unit).
Having an AA unit protecting high value rear line units, is not without it's own risks, bear in mind. That is manpower, pop cap, fuel (and in the case of SU muni), that isn't on the frontline, taking the fight to the enemy.