Well, then several Ripple assets could also be added... As far as I get it, BitShares assets are just reflecting things outsides of BitShares in price, so while they might be capped, they in the end only depend on BitSharesX market caps and are nothing that exists on its own.
Nemstake was listed, wasn't it? so there is a precedent for listing assets.
Now that I'm awake, lol, I'd like to add that although bitUSD is made with BTSX, and shares the same blockchain, it exists independently once an ask or short is fulfilled. Third party exchanges are be able to add it if they want (BTER did, in fact).
Also, with bitUSD on coinmarketcap you can see when the price deviates from a dollar, and see how liquid it is, giving you the opportunity for quick profit.