My apologies, when I was composing the post you hadn't yet posted your counter arguments. It took awhile to capture the screen grabs, crop them, and host them .
As for your first argument, saving polygons might be the reason why it wasn't included, but it doesn't change the fact that he wasn't wearing the sheath. If he wasn't wearing it in one game, it sets the precedent for the outfit to appear without it in another, regardless of whether polygons still need to be saved. It isn't intrinsic to the outfit, so the outfit can exist either with it or without it (for another example, a lot of figurines of Cloud in that outfit don't include the sheath).
And for your second argument, I'll be honest, I don't remember the circumstances surrounding the screenshot I posted. Is there something in particular that happened that would result in him not wearing a sheath, but keep his clothes and motorcycle intact?
EDIT:
The problem with these two examples is that it isn't the outfit in question. Those are both variations of his original clothes from FFVII (which never had a sheath), the outfit with the sheath attached being discussed are his clothes from Advent Children.
What the hell do you want from me? Show me a source outside of FFXIV and Advent Children where Cloud has the weapon rack, please. Because I'm coming up with squat.