Add me to the chorus of "I like to have physical books but prefer paperbacks for practical reasons". The relative flimsiness of paperbacks doesn't matter much: I'm not abusing them, I'm not reading any given book that many times, they'll be fine. But hardback books are bigger and heavier than paperbacks.

If you buy a lot of your books second-hand and live in an expensive part of the world (as I do) then you will likely find that the cost of the books is less than the cost of the larger house you need to fit them in. So you want physically smaller editions of the books if possible.

There are exceptions, of course. Anything you frequently want to have open and flat in front of you -- e.g., cookery books -- you will prefer to be a nice hardback that you can do that with. "Coffee table" books whose whole purpose is to look nice need to be hardbacks (though I personally buy very few of those). Some books never get published in paperback editions.