If you aren't borrowing BITUSD for free on the bitshares DEX you are doing it wrong.
A zero interest loan is provided in order to purchase more assets or they can be used to hedge your assets. Compare zero interest on the BTS DEX with leading brokerages which charge upwards of 10% in order to purchase assets on margin.
A zero interest loan is something only a bank could dream of, and we have that option available to all of us on the bitshares DEX.