This is only good if you are buying Bitcoin when the price is on the floor or experienced correction since you can guarantee that the price will recover immediately to gain profit that can cover you loan interest.
I think the loan interest is not big if we could just pay at the time negotiated with the lender. Also for sure you won't feel those things especially that you are engaging with Bitcoin for long term and not trying to hit up some short time gains with Bitcoin. If you are looking for short term profit then provably that you will chase some profit just to cover up the interest of your borrowed money. What more worse is there is potential that you might lose your money especially if there's some point that you decide to trade your Bitcoin when chasing something and paying to much attention on this matter.
However, if you are loaning money for the sake of long term investment then it’s useless to pay a loan interest while you can just slowly DCA using your salary every month knowing that you can pay your loan outside your Bitcoin investment. You are just taking unnecessary interest while you are just waiting long term for your investment to grow.
No, its not useless since after all you add up some volume for your investment. I understand your point about using your salary but there are some situation that bother's up our accumulation and that one factor could make us decide to loan so that there's no time wasted for those delays happen.
Much better to accumulate slowly using the money that you don’t have any other allocation. Loan will just give you pressure for your investment.
You will only get pressured if you don't have money to pay, but if you have passive income to rely on then for sure you will never think that its bad decision to do it. Especially your intention is to buy Bitcoin and gain some volume.