For short term loans this is usually not a big issue. I think one of the best use-cases are local bitcoin trading (i.e. selling and buying bitcoins on localbitcoins) - sometimes someone wants to buy 5 BTC and the seller has only 3 BTC available - borrowing 2 BTC for a day or two is a nice option, especially if the trader knows it will be easy to convert the fiat into bitcoins again.