Why mark targets?

[YuriRamona]

Marking targets doesn't mean "only damage this target". You can mark a target while still having the DPS dot the unmarked or the (2) and (3) targets or AoE the pack down.

The purpose of marking targets in 4-man dungeons is so that your two DPS don't completely split their non-DoT damage. It simply communicates to the team which mob you intend on building the most aggro on, so they can DPS accordingly. It doesn't say they can't use DoTs or AoE on the other targets.

Yes, this is true. Of course, the DPS are free to DoT even when marks are up. I should have clarified that, for me, marking is almost always equated with "focus down one at a time".

E.g. bees in Sunken Temple, bastards get to do a Final Sting if DPS is split and it becomes stun resistant due to diminishing returns.
Or just killing the little Twintania dragons so they don't spew liquid hell all over the place as long.

Yes, these are good examples where marking is very useful. I would also include Handmaidens (as mentioned by another person here), as well as the big toads in Dzemael/AV.

I'm just trying to speak generally about avoiding marking in most situations as a means to discourage DPS from focusing down one target at a time on ordinary trash enemies.

Even in the examples we have, though, of where focusing down one target at a time is important, it's still arguably important to spread DoTs to the second and third target in that focus list. While the first one will die much more quickly if the DPS don't spread their DoTs to the second and third targets, the total time taken is greatly increased if DoTs are not employed to maximum effect.