Marks are usefull because we can't take hate on all enemy at once easily, and spamming AOE taunts are not really a good way.

I Use marks because it help mono target DPS to know what to hit.

The Tank goal is to avoir the healer to get Aggro because he don't have choice he generate Hate on All ennemies. The more adds in a fight the more I took damage the more the heal I need, the more hate the healer generate.
I can generate more hate than a healer but not the DPS if they DPS like hell. On mono target, the 3rd hit of the aggro combo will generate a LOT but only for one target I have to cycle.

Without numbers I can't keep aggro on Archers or dragoon/monks if I spam flash or Mutilation, if ennemies are a bit resistant.

So it's depends the DPS, the group, if they are strangers or friends... Numbers is an automatism now I use numbers, it's always usefull.