Why do NPCs sleep with their armor on, even though in the CK they clearly do have a slot for sleep clothes? is this some sort of a bug, or did beth disable it? i remember in Oblivion NPCs changing to normal clothes when sleeping..

Probably because it was not a priority one from Beth. (i don't think it's a glitch)

...sure oblivion did that detail right, but i am concerned about other game braking glitches anyway.

or maybe because they want to be ready when a dragon strikes, it's all about dragon(s) isn't ?

The wear sleep outfit in the packages do not work, probably because Bethesda lacked time to make it work. There are lots of details of this kind that have been left alone in this game.