Marking trash is inefficient. Every DD has a high potency DoT, and most of them have AoE attack options. Focusing on a single target at a time ultimately costs more GCDs than killing everything together, and everyone should learn this.
If there is not a very specific, heavy hitting mob with particularly troubling mechanics that needs to be taken out of the picture ten seconds ago, the DDs should be switching to higher health targets as the combat progresses, letting things die to AoEs or DoTs, and tanks should be using AoE enmity generation, even if it does no damage because the DDs will be able to do more damage through DoT and AoE than the tank loses from not doing damage moves.
And if the tanks do leverage their AoE enmity generators, they'll build enough hate while the DDs disperse theirs, and they won't have any hate problems even without tank stance.