calling what the RX-8 has a "useable back seat" is a loose definition and very relative to the fact that the crossfire has no back seat. i rode around in a 40th anniversary edition RX-8 for a week and the backseat really can't hold any people... it is incredibly much a waste of space and seems like an afterthought in the design of the car.
Read this article on the RX-8. It was written by a long time friend of mine...Who also happens to own a crossfire. So he has some insight into what you may be thinking.
http://www.ourtribune.com/article.php?id=3288