Someone is going to have to explain where the interest is coming from? Loans you say? Why would I loan Bitcoins when due to the very deflationary nature advocated i'm likely to have to pay back more value than I borrowed? Accepting borrowing for margin trading (great... just what bitcoin needs, short selling), what would i possibly need a Bitcoin loan for that couldnt be served by a conventional loan? Therefore how is this a killer app?
The traders pay interest on the loans out of their exchange wallet. It is a way to leverage your balance to get a larger return. Any borrowing in any currency requires repayment of a certain percentage over what you borrowed. Have you ever bought a home? A car? Same thing.