Celestial Masters are one of the best mages in the game bar none.

Right near High Seraphs in my book for effectiveness. It's not uncommon in games with me to have 9 out of 10 of the slots filled with CM's laying down the unbridled punishment.

Unfortunately, their weakness can't be averted (I.E. 1 Astral). And when playing against people of any sort of skill, sending out throw away troops to Duel your masters is a common practice. And so either you are forced to take an Astral King/Queen which is dangerous in and of itself, or know that you can't use your mages.