Why does a room have to serve the same purpose it always has?

I just made a recommendation that an unused Dining Room become a Tea Room or a Library. 

In the book I recommended a few posts back, many small Bedrooms were transformed to Changing/Dressing Rooms to accommodate the New York City lifestyles highlighted. 

In my own home, my husband and I office in our large Master Bedroom.  A faux pas to some, I know, but we have worked this way successfully.  This room has the best view! 

How many people really use their formal Dining Rooms or Living Rooms?  It seems like a great improvement to quality of life to repurpose the space to something that could be utilized everyday.  I have seen Offices created out of closets in Unclutterer, a blog I read. How about making a basement into a Media Room?  Common, I suppose.  The photo is the ultimate repurpose.  A spa from a closet!