I love the look of the frameless windows but I would not want a trailer with all frameless windows, so that's a sales turn off for us. We have 2 framed bottom open windows, one small one above the kitchen sink and a large one for our emergency exit. They open about 4 inches. The rest are sliding windows, one nice large one being nearly the length of our couch.
As said the ability to leave them open in the rain is a plus. But the bottom open windows don't hold a candle to the amount of air a sliding window lets in. What I like about ours is that we do have both types and get the best of both worlds.