One important thing I want to emphasize is that, if nothing else, I don't want to have to manually exchange my BTC to bitBTC through some gateway. That is too much work. I just want to send BTC to the bitshares exchange trade it, and then send it directly to my BTC wallet when I am done. Maybe integrate a BTC wallet into the client? I don't know, just trying to see a way to make it work. I can tell you I would definitely use the service if it were available. I HATE having to trust my bitcoin on regular exchanges.
I hope coinbase or the winklevoss don't add pegged assets or something similar into their exchanges, because those I would trust to use and they would effectively have all the same functionality as the bitshares exchange. Don't put too much importance on the decentralize aspect of bitshares, most people just want to trade what they want to trade, when they want to trade it, quickly and in a safe and easy way whether it's decentralized or not. It's the same concept as what bytemaster says about the light wallet, most people won't care that it is less decentralized.