I've never run into the problem myself, but I think the main issue comes from the increasing discrepancy in attack power between [standard] tanks and dps. Each ilvl increase is giving dps a much larger increase in AP than the standard vit build tank, and their enmity modifiers just can't keep up with the difference in damage output. If Tanks' AP was to be modified by vit rather than str, this wouldn't be an issue and tanking accessories would be as powerful as their dps/healing counterparts.