That's right! Defenders should get an exotic armor piece that grants armor lock at the cost of melee energy.
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I remember a guy chasing me in Reach with his Rocket Launcher. I went up a ramp and turned into a little building. Knowing that the guy was still right behind me, I went into Armor Lock as soon as he rounded the corner after me. He fired his rocket, but it bounced off me and killed him with a direct hit. Oh, that was a glorious T-bagging...