Open Range was the first place I saw the big U lounge / dinette. Getting rid of the separate couch for one big unit.
A key part of that type layout is removable tables that can be setup and take down fairly quickly.
Not wasting space on a dinette to give more lounge space.
The Open Range units I've seen have a bed in the center which folds out. So you no longer have a sleeping option of the dinette for small children and a jackknife sofa for larger kids. Just one double or queen size bed.
Better for adults, not better for several small kids.
Open Range puts them in the bunkhouse models because they don't expect the customers to need the small kids sleeping options with the bunks in back.
As far as comfort. They are better than a jackknife sofa, and most regular sofas with an air mattress fold out. You can lie back and stretch out because of their length.
Personally I prefer recliners and a stand alone table with chairs, but that's not an option due to limited space in those models. The bunkhouse model trades a lot of living room / kitchen space for the bunk room.
In my opinion one big U which can be a lounge or can be a dinette is a better choice than a small dinette and a small couch.