This might be controversial, but i like playing barbie doll with my weapons rather than having new expensive skins added.
Currently most weapons have 3 parts per category. If they added even just one more part in all categories for a weapon that massively increases the permutations of possible weapon creations.
With weapon skins, if i dislike one part of it then i'm not gonna use it. With the default weapon system i can at least customize the parts to get rid of the portion of the weapon that looks ugly to me.