Why you should borrow w bitcoin? For the same reason that you borrow fiat, you should take a loan if you have an idea for a productive activity but you dont have the bitcoin at hand.
like borrowing bitcoin inorder to invest to a coin which you know can skyrocket in a week.
and then payback with interest after you withdraw and profit. opportunities like these are suppose to be grab. If I only knew monero will go that high before a week ago, i might just sold the rest of my XAURUM and LTC to invest to it.
Ya for that you can do as this will be good, but what if that coin dont skyrocket and it fell below the purchase price, so their is always both side. If you want to borrow or get loan, then you should also have a valid collateral then only the lender will give you the loan as he also need the security.
That is why i said "IF I only know".. which means you would have to have inside information and to what is gonna happen in the next few days.
Let's say the Asian countries will soon be fighting over a disputed territories and war is almost going to happen in days. This is going to be an indication that JPY might just drop again and so the traders will soon go short on USD/JPY.