For many modders, let's say someone building a starwars mod, they want to put in an Xwing and tie fighter hull. It doesn't matter so much if they can tweak what that ship can do or can't do, though that would be nice, I understand for balance reasons it's difficult. It's more about making it look like the right kind of fighter for that race. Even a completely unique race, is probably going to want a certain type of look to their fighter.
One way of doing it, is having the weapons and components a fixed list, while having the hull variable.