Poloniex just proved us they don't run a fractional reserve for BitShares at least. What if the exchange you used could do this for whatever coins/assets you hold? Wouldn't you feel more safe and have more trust on their service?
Other exchanges can do the same. There are currently a few projects and exchanges working with OpenLedger to bring a better service to their costumers, this includes speed, safety, more market depth and overall performance.
Wouldn't you like if exchanges were more transparent and you could own the private keys to your funds? More info here if you're interested: