Why borrow at all? If he's making a narrow profit margin, why does he bother taking all the risk? If he's making a decent profit, why not use that profit to pay down debt?
Puzzle out why a bank might pay you interest on the money you deposit on account with them. Now reflect that answer back to the pre-FDIC days. Is your answer still valid? If so, I'm guessing you have your answer as to why Patrick would pay interest to his depositors. (hint: it optimizes his personal TVM)
I remain keenly interested in your reply to BurtW's question explicitly addressed to you. Is there any single economic activity you would deem rational for someone who is long bitcoin?