Becuase 5mans don't hit as hard as raid bosses. The amount of rage you get depends on how hard the hit you take is. If i'm tanking an HC I generally will throw on my arms gear, a hit mastery 1hander and a hit mastery shield, generally i'll only have a little less armour, less avoidance and less block and a little less healh, (but the 15% buff counters that), i take more damage, get more rage, & vengeance stacks quicker. Being hit and expertise capped means i get rage from white swings a littlemore regularly and i'm gauranteed my abilites will land. generall pulling about 14k dps on things like the first boss of lost city.