I think the paradigm you're missing is that this is a decentralized derivatives exchange. bitPTS is designed to track the value of PTS but it is not itself PTS. bitPTS is created by shorting bitPTS and backing it with 2x the collateral in BTSX.
The idea being that you can store your wealth in whatever asset you feel has either the most stability or best chance for growth vs another asset. So, for example, if a snapshot is announced for PTS and the value starts to rise your bitPTS that tracks the value of PTS will rise in relation to BTSX. When you want to cash out to PTS on the BitShares-PTS blockchain you would sell your bitPTS for BTSX, send them to BTER (or other exchange that supports BTSX) and trade them for PTS and transfer them out to your wallet.
So the next obvious question is: Why bother? The biggest advantage to storing/trading the bitAsset is that your wealth always stays in your wallet, not in an exchange's wallet (see Mt. Gox for why this is huge). You can freely move your wealth between bitAssets all within your wallet. You can also earn Interest (dividends) on bitUSD starting tomorrow. The Interest is profit sharing - transaction fees that normally go to profit for a centralized exchange are distributed to bitUSD owners instead.