AIUI it's basically a trade-off between body rigidity and capacity. The more you rigidly brace across between the two suspension turrets, the stiffer the bodyshell is (for a given weight), but the smaller items you can carry.
For example the FD3S and Z33 are both 'hatchbacks', but both have fixed braces joining the tops of the rear suspension turrets. Neither is very useable in terms of boot space. Yesterday I just managed to squeeze my wife's smaller bicycle with front wheel removed into the 350Z, so I could drop it at the dealer for a service, but it wasn't easy to get it in, and the bike was also a bit small for me to ride to work ;-) The 180SX had no such brace and was pretty good for carrying bicycles, but was less rigid.
In some ways if you're going to brace it anyway you may as well just have a separate boot with a ski hatch, I guess, as on the GT86.