I think Walt in the past has stated his belief that interior weapons and shields lowers the creativity in this game, as it would cause people to just build big, boring, armored boxes with weapons, shields and thrusters built inside.
I think he's opened up the possibility of certain special weapons and parts (ex. the Tractor Beam) being roof mounted, but that they would have some major drawbacks to prevent spam. I think there is (or at least was) a "super cannon" on the Trello that was roof mounted.