A problem now arises with the other classes of the trinity, and no longer a problem with Tanks.
See, according to the original proposition you mentioned, all damage to a boss or mob would be completely dependent upon the DPS. In which case two perfect tanks, two perfect healers, and three perfect DPS-es would be undone by one suboptimal DPS (because the damage required by 4 DPS may not be enough with 3 good/1bad DPS)
The fact that the tanks and healers can bring something to the table allows for flexibility in a party to have subpar members.
If a healer is terrific at their job, they dont have to sit there casting cure over and over..... which is what I see all too often in dungeons. Rather, the healer can help out a DPS who decides to get face-lasered by the boss, or attacking adds that spawn in a fight.
If a tank is terrific at their job (which is not hard at all in the least. A tank who strictly only uses the butcher's block combo should never, under any circumstances, ever lose hate), they should be able to do something else other than tank. Which includes using their own buffs to help the healer, or putting in their DPS to help the party overall.
If Im doing Bismarck EX, and the party is standing on its back shaking violently about to be thrown off, I as a tank would want to be able to use Fell Cleave in a pinch to push in that last bit of DPS to win.
Not solely rely on my four DPS, hoping they mastered their rotations.
All the people you see tanking in Deliverance stance are the ones who have mastered their job. They have learned how to push their job to the limit by keeping hate and contributing to the overall party DPS.