Actually it's the opposite. The costs people can't recognize are the massive opportunity costs of having rooms that they don't use 99% of the time. (DISCLAIMER: I am not talking about the people who do use them! Don't bring up your cousin Bob who hosts events all the time and think that you are refuting me.)

This is because people look at the utility of the room when it is used, but forget to multiply that by the frequency of its use.

And yes, sometimes a dining room can surprisingly come in handy for something that comes up. However, this is true of almost any use you can think of for the room. I would venture to say that most people who have dining rooms, who don't routinely host dinners, would be better served making it a video game room, or an aquarium, or a giant closet, or a room for whatever hobby they actually do. Yet across America, millions of dining rooms sit empty for months at a spread, screaming to be made into something that more accurately reflects the lifestyles of their owners.