When you parry an attack, your next auto attack happens sooner. that is parryhaste. It was removed from bosses a long time ago, but still exists for players. This means than parry increase SoB generation both by the fact that you parried (the new change in 5.4) and the fact that your next auto attack will happen sooner. This means more resources, which outweighs the significantly small effects of DR on avoidance.