it's a good point, i think if you want a coupe you can see other options on the market. If i were to go for a coupe I might as well go for 3 series coupe. Z4 makes more sense to me as a convertible, as a purchase.
design wise there is no doubt with roof up, the coupe looks a better design. The hardtop make a little bubble, kinda like canopy on a jet. It's not the best look. Roof down the Z4 is wicked good.