This isn't a Miata hardtop coupe, it's another convertible version of the Miata with a folding hardtop. It's a lot cheaper and easier to make a folding top out of cloth or vinyl than it is to make one out of pieces of sheetmetal.

If you're referring to cars like the BMW Z4 M Coupe and Porsche Cayman, they offer increased performance over the convertibles they're based on.