If you take the definition of ownership of a coin to mean you holding the private keys, then Bitshares is NOT an exchange at all, but just a derivative play on the price of an underlying that you hope will be maintained through thick and thin. You cannot actually trade BTC on the Bitshares network. You can trade BitBTC that you assume will be equivalent to the price of BTC or that you can get exposure to BTC by owning bitBTC but that doesn't mean it is a decentralized exchange for BTC. Not saying this isn't something nice or something that will have a lot of demand, but calling it a decentralized exchange would be wrong.
I understand what you are saying and I agree that it is not a decentralized exchange in the true sense of the term. I guess I am using that term loosely because it is the closest thing to a decentralized exchange that actually exists (to my knowledge.) It is not as good as a decentralized exchange in that you don't own the private keys to the crypto you are trading and there is added risk involved with that, but then it is also better in that it allows the trading of commodities like gold and oil.
I guess it depends on your risk tolerance at this point, and I agree BTSX is a risky investment (what Cryptocurrency isn't really.) I think if BitsharesX stands the test of time I think that will go a long ways towards instilling confidence in the market and bitassets. Thus increasing utility, liquidity, and the value of BTSX. Buying BTSX now is a play on all of that working itself out, and the developers/community figuring out how to ensure the market pegging mechanism is as accurate as possible.
There is a lot of discussion being had as to ways to improve accuracy of the market peg without putting too many restrictions on the market or centralizing it. I think if the market peg can be made as accurate as possible, the infrastructure surrounding bitassets will fill in and they will have just as much utility as Bitcoin through the use of multi coin payment processors and payment gateways. In that case, it will be similar to owning private keys except with a little added risk that would arguably be less than the risk of trading on a centralized exchange.