ZAFT was probably just test-bedding the technology of reconfiguring packs. That is the basis they picked the Zaku with its wizard packs. They're probably experimenting which are suitable for mass production and compatability stuff. As for the Destiny Impulse, i believe that was the precursor to the actual Destiny, Impluses' Deuterion energy supply probably coudln't handle all the new tech., Heck even the current Destiny Gundam has trouble in terms of endurance it seems. lol major power issues.