The tradeoff on short domestic flights is that it encourages more - and larger - carry-ons, which slows down boarding/deplaning and therefore adds to turnaround time. If I don't have to pay for checked bags, I'd often prefer to have mine checked, especially if I have a connection - but since I do, I'll squeeze everything into a carry-on roller bag instead. Personally, it only takes me an extra second or two, but when you have a whole family doing this and only parent who can actually reach the overhead bins, it bogs down the whole aisle.