I was recently at my brother-in-law's house in suburban Boston. It is a large house with all the typical rooms desired by people these days.(4 bedrooms, den, living room, great room, kitchen with large breakfast area, bathrooms everyplace, and basement rec-room)
It struck me that all my suburban friends and relatives own houses like this that have two underused rooms but that none of them would do without. These would be the Living Room and the Dining Room. Over the years these rooms have slowly been downgraded and relegated to less traveled parts of the house. The living room has been replaced by the Great Room which is usually very large with cathedral ceiling and fireplace. The Dining Room has been replaced with the breakfast area. This area started out as a nook and has gradually grown quite large. It is often a buffer between the Kitchen and the Great Room.
My brother-in-law furnishes his Living Room with some formal and fairly uninviting furniture and a piano that no one plays. It is a small square boring room with only one small window. The Dining Room is a mirror of the Living Room on the other side of the front entry vestibule (another underused space since people mostly enter through the garage). they have turned the Dining Room into a Play Room for their two kids.
I believe this trend is due to the less and less formal way we like to live these days. These rooms are like organs in our body which through evolution no longer serve a purpose (like the appendix or the small toe). Will there come a day when developers will get up enough courage to eliminate them?