They're fine when you've got a huge amount of square footage to spare, for the number of people in the house. A common time to realize how much nicer some walls and doors would be is after having kids. Walls and doors let you have a larger number of occupants living in a given space without sacrificing comfort.
Our house has an open kitchen + living room + entryway + stairway, and that's not so bad—because we also have a huge basement, four bedrooms for 2 adults + 3 kids, two extra rooms that are walled off with doors (we had to add the doors, though...), et c. So that open space is OK, because we have way more house than we'd need to be similarly-comfortable if it were better laid out, including having more walls and doors.
Our house has an open kitchen + living room + entryway + stairway, and that's not so bad—because we also have a huge basement, four bedrooms for 2 adults + 3 kids, two extra rooms that are walled off with doors (we had to add the doors, though...), et c. So that open space is OK, because we have way more house than we'd need to be similarly-comfortable if it were better laid out, including having more walls and doors.