You shouldn't be able to get good AR from mixing pieces.
The various weights of the different kinds of pieces would make you unbalanced, they probably wouldn't fit together in the first place, and even if they did, it would logically leave tons of gaps in the armor the full set wouldn't have.
You aren't supposed to "customize" your armor in an RPG, your supposed to use the best stuff you can find, and that is, and always has been, armor sets.
Well
A. Oblivion and Skyrim dont have "pajama" one slot armor/clothing, they had helms, body, gloves, boots, and shields. So I don't know where you keep pulling that from.
B. Yes, Oblivion, Skyrim, and Fallout 3 were miles more balanced then Morrowind becuase they had less pieces.