Armor of Burden is a harmful type of runic armor. Each time you are hit while wearing it, there is a chance that the strength requirement of the armor will increase by one. Eventually, the armor will become so heavy that your defence drops to zero, allowing almost any creature to hit you automatically.
Unlike some other negative runics, armors of burden has no tactical value whatsoever. Unless they can readily be negated in an armor-starved dungeon, there's no functional reason for keeping them around.