Well, with Comets Call being able to hit any unit in the game, he needs a close-up mega-damage spell. I agree that it would be op, but... well, Kroak isn't exactly a weakling anyway, is he? Does he really need it?
Well, he doen't need it, but actually Celestial Deliverance got a max range of 3d6+8, so about 19" (basically the range of a standard spell). With AV that would be 34"... for three castings. No, thanks, I like to keep my friends.