A) Those little glowing things around your Spartan and Elites. And, if you are to go off Halo Wars, some vehicles. B) It would take longer to kill them, and being that this includes the Covenant, they would be facing a group with much better armor, too. I don't recall Stormtrooer armor actually serving a purpose in terms of defense. C) So the Empire lost due to its own idiot soldiers? They already can barely hit a Rebel, let alone Luke.