[Kuritza]

>> These are the numbers to look at.

The numbers lie. I am not sure why, but astral spells with some penetration items work FAR better than beforementioned 4%. In my case it took exactly 3 castings to pass through 25 MR, supposedly without any penetration items on casting mages. Heck, I routinely have my 25-MR SCs paralyzed with the very first casting for 40+ turns, and I have given up hope of ever using a SC against R'lyeh.
Your 16 attempts number looks really, really overestimated.

And in case of Caelum you can count the turns you have to spend recruiting priests for calling back your God instead of recruiting mages and the losses your army will suffer without him now that your God doesnt stand between your mages and enemy sacreds. Probably you will simply lose the game.
Ah, and the mage-turns for equipping your god again.
Oh, and why bother calling him, he still has all these kills on him and will eat VotD again.

And one more thing to consider. This is not just a highly situational SC-removal spell. Most mages dont stand a chance against 40-50 zombies they will likely have to face alone, which is a very good result for 3 astral pearls.