And even with the itemization patch that's supposed to allow current low level legendaries to roll at higher item levels, who's to say that the stats will correlate to the looks? Stats are more objective in a sense that you can see tangible benefits from equipping one item to the next depending on your build. However, cosmetics are more subjective.
So why not have a cosmetic item slot in which you have your primary slot for the equipment that you want the stats of while your character displays the look of the item in the cosmetic slot? I'm pretty sure I've seen this done in other games (though the names of any escape me at this hour) and it works pretty well.
The only caveat should be that the cosmetic weapon would have to match the weapon type of the equipped (ie no using a 2H cosmetic weapon while a 1H is equipped, sword should be paired with sword, mace with mace, bow with bow, etc).