As a tank you can focus on DPS is because your teammates are good, they kill fast and deal with the situation quicker, not because the tank is good.

But a lot of solo players tank may do not have luxury of a good premade party, they have to find a proper way to keep a random party safe and try to cover their mistake, which means more time in tank stance. In a random party the tank is nearly always the default leader. These kind of player to me is a real good tank.

There is a little difference between "dpsing at maximum possible and use CD when necessary" and "being a tank while try to put out maximum dps".

Personal judgement as both tank/healer player, I really can't see my hp drop under 50% and see my tank have less than 50%