I am assuming that the Marauder is relatively geared (i90), it shouldn't be a problem if he's taking 25% more damage as the mobs isn't that painful to him in the first place. WAR/MRD's ability to self-heal is pretty high. I don't see any issue why using Blood for Blood as a MRD is "wrong". I wouldn't say that a MRD/WAR's job is to DPS and not tank, but if the situation allows the MRD to dps and tank at the same, why not?