Right now we have a bedroom not being used. Its ds's bedroom and once he and gf get married, I will transform it into something else (not sure what yet, maybe an office/exercise room). I won't do it until he is actually married because the last time I thought he had moved out for good and did something with the room; he moved back!!
We have a family room and a living room--not formal, just a living room, even though our house is really not that big. Both are used pretty much all the time. I wanted both rooms so the kids would have a place to entertain friends without us having to be right on top of them.
What we don't have is a dining room. Our family room and kitchen are all a large open area and the table is kind of just between the two. When we designed and built the house, I specified no dining room; it would just be wasted space. The contractor kept wanting me to make the living room into a dining room, but I like it much better the way it is.