Although they are not backed by the assets that they are expected to track, an incentive might exist for at least some suppliers to accept BitAssets in 1:1 exchange for the assets that they represent.
Imagine, for example, that someone issued BitGreenArabicaCoffee, that its price in terms of one's favorite numeraire were X, and that the delivery price asked by coffee wholesalers for green Arabica coffee were approximately X.
Somewhere in the world, a coffee grower might be able to deliver green Arabica coffee at a price enough less than X for it to be worthwhile to accept, e.g., one ton worth of BitGreenArabicaCoffee, immediately sell it on the open market, and ship the coffee to the buyer.
Let us say, hypothetically, that the total quantity of BitGreenArabicaCoffee in circulation were a fraction of the total worldwide Arabica crop. In this way, it might feel as though BitGreenArabicaCoffee were backed, even though it were not.