DPS Warriors DO give something Protection Warriors don't - physical vulnerability. Prot Warriors haven't had access to Colossus Smash since 5.0 released.
Frost/Unholy DKs bring PhysVul as well.
If they are trying to fit in every buff a rogue+DK is pretty redundant.
Looks like the OP (at least going off of the Qon/Consorts log) has a Feral, DK, and hunter as physical DPS. Your hunter also had a hyena out so you were down a buff anyways (spell haste; bring a spore bat for the two casters + healers). If anything, you should be replacing the Feral since there's a mage in that 10m group and they bring the crit Ferals also bring. Seeing as they're also an officer though, that's not likely to happen.
Since the hunter was already bringing attack speed, there was no reason you couldn't have sat in for the DK.
So yeah, could just be bring the player not the class.
In any case, we do bring more raid utility than a DK. Their only raid CD (that's useful on a boss) is going to be AMZ, and that's pretty pitiful since it lasts maybe a tick of anything. We've got Cry + Demo along with Skull Banner, we're far more mobile than a DK, etc. The 10% attack speed is rarely going to make or break a fight, especially since it seems that normally the OP has a protadin + warrior, and I don't think basic attack speed affects Sanctity of Battle, just haste.