This is merely random speculation, but I got to thinking that the most successful MS seemed to use shields. Every (or at least, almost every) Gundam unit has had a shield of some sort, though most Zeon MS (this aslo includes pretty much every anti earth faction) has lacked shields and well, a long lifespan as well. Though most suits labled as "enemy" wont live long anyways, simply due to the whims and thought processes of the creators, it seems the suits with shields survived slightly longer. Now I knowthe Zaku II technically had a shield, but I'm reffering to "hand held" shields more so than the shoulder mounted shield of the Zaku II. Anyways comment back and tell me what you think.