It REALLY sucks when you aggregate a rare drop to an item that you also modify with stats and what-not(even more than doing goblins every day I daresay :O). Then you need to go through the whole process of getting it again. For instance, I got strawberry aggs for my hammers, but since I got a t9 one cheapish I need to find another one next year (or buy it from someone).
That was the point of costumes right? Should be the same with weapons
For why they haven't done this, is it because of the issue where in the animation you hold the weapon with 1hand or 2hand? There could be a way to exchange your weapon for the 1handed or 2handed version