I think the rear seats were added to make insurance cheaper in America?
They are useless for passengers (except small children) but I use them all the time for shopping and would not mind placing luggage there.
I got the car because I wanted a large, powerful and comfortable 2-seater convertible car - luggage space was not an issue.
I looked at newish MX5's, they look great but are tiny and difficult to get in and out of.
So pleased with my SC430.