Well in my design BTSX would get destroyed when converted to BitUSD instead of used as a margin. So create/destroy all you want, but the block chain is the market maker and all the BTSX holders will gain from the conversions back and forth.
If you have a trusted source of prices and a trustable way of getting that price information into a DAC (median delegate feed) then I suppose you can get rid of the entire Bid/Ask system all together and simply have the DAC create / destroy XTS as necessary for people to convert into and out of USD.
However, if you do not have a price source (all exchanges are shut down by government) then it becomes a bit more difficult to know the "true" price upon which to execute your automatic process.
Also by having the network automatically create/destroy XTS "on demand" for conversion to/from BitUSD you are forcing the shareholders to take the full risk of being short.
That said I really like the idea for its simplicity and clarity. It would not have been possible prior to DPOS, but now is potentially viable.