I would love a way to reuse an appearance of an item I already have without destroying it. I have 2 sets of armor and 4 daggers for use with different stat sets. I For expensive appearance item (say, T3 cultural armor), it is unlikely that someone would get two sets of it. When swapping to another stat set, the player is faced with either finding another appearance to go with or a very large expense (either buy another cultural armor or destroy your exotics). Right now I think almost everyone goes for the former.
To mitigate this:
A good addition to the gem store would be an item that could swap the appearance of two items. This could be paired with an item that removes upgrades from items or swaps upgrades between two items. Putting both of these features into one item might be confusing.