I'm PS4 player and I don't have space for mark numbers in hotbar nor have I fingers fast enough to set them in the middle of a fight so I just tank without marking and switch targets constantly trying to keep aggro on all targets at the same time. I think as the one blm mentioned before - players able to deal massive damage usually are capable to notice what tank can and can't do and adapt.
I have had dungeons where whole run both dps will always attack fifferent targets but those usually are brain dead people that can't do enough damage for me to lose aggro. Smart and skilled, the truly powerful dps just quickly notice that tank can't keep aggro and adapt to tank playstile very fast.