So if I have have 300$ available and I want to buy Bitcoins for 300$. I would rather not use leverage and put all my 300$, rather than putting only 100$ with margin at leverage 1:3. Right? Because as long as taker/maker fees are concerned, both actions are equivalent, but with leverage, I would also pay interest fees. So leverage ends up being more expensive.
Absolutely. There is no reason to borrow something you already have.
As far as I understand, the only two situations where leverage makes sense, is 1) I want more buying power than what I have available, 2) I predict a Bitcoin drop and would like to sell but I have no Bitcoins.
That's right, maybe add 3) You want to keep some buying power in hand if you see another trade, like buying some ETH or LTC, and wouldn't want to have to exit the BTC long trade to do it.