What are the reasons someone would buy/short BitBTC as opposed to buying/shorting the real thing?
Because could do so without going through a trusted third party (an exchange), where you are instead exchanging btsxIOUs for xbtIOUs, and your funds could theoretically be seized, stolen, or held until you comply with some sort of kyc (know your customer) procedure.
It is safer than trading on an exchange if you are trading just for the sake of trading. If you are trading to spend them then that is another story, but I have a feeling the bitasset infrastructure will fall into place to allow people to do that.