So the way I understand Bitshares is it is a way for a person to put up money betting on a short, and then others can accept on the other position and try to go long.
Yes, but the whole system in its entirety has many benefits rather than just speculating on price movements.
Of course it allows people to make money by going short or long by speculating what the price of an asset and collateral will do.
bitFIAT is a good hedge against crypto bubbles.
It allows people to day trade or diversify their portfolio with cryptocoins, FIAT, stock, and commodities (gold, silver, oil, wheat, corn, etc) with no counterparty risk due to trusted third parties (centralized exchanges).
It allows someone that likes the idea of decentralized cryptocurrencies but doesn't like the volatility of them to still use them by storing their cryptocurrency in a more stable asset (bitFIAT).
It allows Bitassets to gain interest from trading fees.
It allows BTSX stake holders to earn dividends by the destruction of transaction fees. No more BTSX will be printed, so by destroying transaction fees it makes the cryptocurrency deflationary. It is one of the only cryptocoins that is truly deflationary, as Bitcoin/etc is still in its inflationary stage and once all Bitcoin's are mined the transaction fees are paid to miners and not destroyed.
You can hold your Bitcoins (or other slower cryptocoins) in bitBTC, earn interest, and have the benefits of an average of 5 second confirmation times that have more security of a 10 minute Bitcoin confirmation. DPoS is a fast and efficient consensus algorithm with 10 second block times which is convenient. Admittedly the infrastructure and payment processors aren't there yet... but this is a future benefit of the system and you can already do this with person to person transactions.
Hmm.. I'm probably forgetting something.