First off, thanks for telling me I need to learn to play.
Secondly: Marauders in a sense have more reduction that heavy armor classes considering cloak of pain gives them an additional 20% dr, which includes elemental / internal damage.
Lastly: Undying rage is perhaps the best CD in the game, considering it's 99% dr. So yes, being almost invulnerable for five seconds is kind of a big deal, when it's on such a short cooldown with set bonus etc.