Why a tank need to dps at all?

[Felis]

And dealing damage reduces the damage people take by making the mob die faster, thus functions like a shield.
Moreover, you do not need to increase your enmity when you already have aggro, just as you do not need to heal when nobody is hurt.

Let's not start the comparisons and keep trolling, aye?

tanks can still lose aggro, if he build up less enmity than the dps and healers during the fight