It is never useful to borrow money or bitcoins as you have to give back more
But there are events that borrowing is going to be useful at all. Just like if you are a trader and you don't have enough bitcoins to your wallet address.
And you are still waiting for your earnings and there is an ICO that you want to invest some bitcoins. Then what you are going to do is going to borrow because you know that you are going to earn with it.
Well thats a good intention too when its useful to borrow bitcoin but still it has risk even you are relying on ICO you wont know on what would be the price when it launches when you are lucky or that crypto would sell high then you would benefited and can repay your loan in no time but if not then you would pay add up some amount to the borrowed money.