Okay, I kinda see where you're going with this. If you're playing Aegislash defensively, then King's Shield could be useful to change back into shield form after every attack. Low speed IVs and a speed reducing nature makes you go last almost every time (so you take the opponents attack while in shield form) and Leftovers slowly heals any damage taken. I personally prefer the hyper-offensive route, which is to set up as many Swords dances as my opponent will let me and then spam Shadow Sneak to death (sure, eventually Aegi will faint, but not before taking out half of the opponent's Pokemon). I guess it just didn't occur to me that some people prefer to play it safe. Still, using King's Shield can give your opponent a free turn to raise its own stats and cause massive damage to Aegi, even in shield form.
edit: switching in and out also changes you back to shield form.