Why mark targets?

[YuriRamona]

Second, you are in fact fundamentally wrong about the DPS of dots and AoEs. For melee, at least. For both monk and dragoon one of our DoTs is in the tail end of a combo, DoTs also take far too long to tick down completely, more than the encounter should last after the application of the first comboless one so there would be a long stretch of time where we should use our AoE attacks.
Also, AoE attacks consume a LOT of TP for melee, we run dry extremely fast using to the it's impossible to do two pulls back to back, and for both monk and dragoon our most TP-efficient and powerful AoE is ALSO part of a combo, so we need a single-target attack before moving into AoE, thus we'd like to focus-fire that too.

Obviously, the enemy will die before the DoT ticks to full if you're focusing down one at a time. If you're applying DoTs evenly across enemies and also spreading your single-target weaponskills, then almost all enemies will take longer than 18-30 seconds to fall.

I see this contradiction all the time. A common excuse is "well, the enemy dies in less than 18 seconds anyway, so DoT is not worth it." Yes, but this is only the case if you only use single-target attacks on one enemy at a time.

And yes, AoE abilities are not very TP-efficient. However, single-target combos are only second on the list in terms of TP-efficiency. DoTs are the most TP-efficient, bar none.

[WellFooled]

If you're applying DoTs evenly across enemies and also spreading your single-target weaponskills, then almost all enemies will take longer than 18-30 seconds to fall.
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In this case you may be maximizing damage, but at the expense of other things. For example, when you kill a monster it can no longer deal damage to you, which means the healer and tank's jobs gets easier. However if you deal equal amounts of damage to all monsters you'll have to put up with the max amount of damage sustained for the entire duration of the pull.

There are also many situations where an enemy group isn't sitting in a tiny little compact space. You've got ranged monster hitting you from afar. To spread dots and single target weaponskills out evenly would require the melee dps to run from target to target after every GCD, which would ruin your dps.

There are also many situations where a single monster poses way more of a threat (or maybe just an annoyance) than other monsters (like the succubus in AK, who can hit you with powerful AoE magic) and should be killed so that threat is eliminated. This is also the case in dozens of boss fights. If you don't kill the adds, they'll eventually overwhelm you.

[Viridiana]

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1) That's cool, but the OP already said he only meant for this to apply when incoming damage is not a concern.

2) Doesn't apply to BRD, so you agree BRD should always multi-dot? And that melee should multi-dot if the mobs are close together?

3) Those monsters are often still part of a pack. Kill them, then spread dots over the rest of the pack. BTW, the Succubi in AK should never even touch you. . .