Monsters are made to do monster-y things. They get attack/casting animations, things to do when taking damage like flinch, block/parry if applicable, walking, running, and then some. Fundamentally, this is more useful to the game as a more robust monster variety leads to a broader playing experience. All this system does is put players in the monster's skin, which also requires all those animations that have been already created. There's no gear swapping to blink you out because mechanically our player model "doesn't exist" at that moment in time.
Now, some helmets did take hair style into consideration, but it was basically along the lines of "If ponytail is covered, don't render the ponytail" line of thinking. These conditions had to be applied to every piece of headgear with respect to that particular model. If not, you'd get clipping issues, or worse, invisible heads. Could some helmets have fluffy tails not unlike how some chest armor drapes over our legs? Sure. Might this be a polygon issue, though? I don't know. We've yet to really see capes, either.
Random combat-capable costumes just leads me to a simple question, though. Why? What does it accomplish that us in our gear doesn't? What would you actually create? Truth be told, a lot of XI NPCs just borrow our armor resources glued together into a single DAT with some common animation data. And this is something anyone who's played around with any kind of XI ModelViewer and paid attention would've learned. As is, playing as a monster offers a wide variety of skins to inhabit. In the future, if they decide, "Hey, you can play a Mithran Sin Hunter!" then they can add that to the system, too. It really is a simple matter of them recycling materials proven to work instead of spending all this time making new **** just to find out it might make the PS2 explode.
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Violence good. Sexy bad. Yay America.