Nope.
Of all the excuses they gave for the merge (more NPCs on the screen, better designed armor, loads of smithing options to make armor look different instead, etc) none of them actually made me think "this was a good tradeoff" when playing the game.
I still think the main reason they did it was to further streamline the game and make it more accessible. After all... the average non-RPG fan might find lots of armor pieces overwhelming...
Of the excuses
1) is stupid only time Skyrim has more people on screen than Oblivion might be the civil war, solution is simple make the soldier uniform one piece not all outfits. Non used item slots does not use resources Skyrim has lots of hidden ones moders use.
2) As far as I know we can not smith different armors variations.
3) to some degree, however this would just be a couple of armors who would get problems so make them one piece, multiple armors and clothing in Oblivion was one piece
4 Yes most casual players get dumb-folded of the concept of pants.
WOW is also an causal game, Sims is even more and both has many more equipment slots.
More probably they got lazy, its easier to just make one piece armor, they still managed to get more clipping issues than Oblivion had with boots and gloves.