An AR with a carbine stock and an unstaked castle nut is indicative of someone who thinks they are 'smarter' than the engineer who wrote the book. It's also a sign that the AR owner is either ignorant, stupid or just lazy or some combination of the three.
Castle nut comes lose = stock becomes lose, turns during operation and can make the AR inoperable, buffer retainer and spring auto eject into the upper and lower locking up the gun and normal clearance drills don't get firearm working.
End result = a bad day at the range. A 'THAT GUY' moment during a training class. Losing a match/stage. Or worse, your death because your firearm didn't work in the heat of the moment and you didn't have a secondary.