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[Gserpent]

This really isn't a winning argument in the context of CC. It's the farthest thing imaginable from an attrition model.
With the ready availability of healing resources to all jobs, non-lethal damage in combat has very little opportunity cost associated with it,
and non-lethal damage outside of combat has no opportunity cost whatsoever.
Dealing 36k damage to one target is better than doing 72k spread over 3 targets.
Dealing 36k damage in 6s and then doing nothing for 6s because your target is dead is better than doing 54k damage over 12s.
You can top damage every match playing Fire BLM, and the meter will keep telling you the losses weren't your fault.

What are you talking about? Healing is a huge opportunity cost. Target priority in CC is pretty much always "the person without any mana left" because you know they can't heal much and heals from healers and other classes are more like backwards damage mitigation than actual healing. If DoT's and other incidental damage cause someone to heal, that means they're at 3/4 instead of 4/4. Bursting people down with CC is unreliable because of Guard and Purify (although server tickrate and latency can get you kills... christ knows I've died a few times while mashing Guard only for it to just not activate until after I was dead... instance servers seem to struggle with handling the number of games running during prime time on weekends), so it tends to be about attrition. Against good players, anyway. CC'ing someone is generally how you can force out a bunch of heals in quick succession, but a 25k DoT will leave a lot of classes at less than half HP if they don't heal.

Sure, you can pop a potion, but that's a 5 sec cast on top of needing to move to a place where it won't be interrupted, and then moving back to the fight. That's obviously a lot of time to make a move, and DoT effects tend to have 100% uptime if you want it. People undervalue DoT's and discard it as damage padding because they don't really grasp how much CC is actually about attrition. Burst doesn't work if the players on the other team are paying attention and are competent. If it did, 5 DPS teams would be dominant... but instead they tend to lose quite badly to more balanced teams, and 3-healer teams (particularly SCH AST SGE) are actually extremely strong because of their ability to mitigate and cause attrition.

[Lymberey]

a 25k DoT will leave a lot of classes at less than half HP if they don't heal.

I believe the highest overall potency DoT without LB is 22.5K which is 5 ticks, 4.5K/tick. In 15 seconds, the probability of someone not healing is practically zero, and 15 seconds is a very long time for how fast CC gameplay is. Against competent players, it is much more manageable than burst and cc as they already know when and how to engage and disengage.

More importantly, 15 seconds is a very long time for Mana to regen which is 500/tick in total of 5 ticks. So, that 22.5K DoT is actually almost free to heal though. Any DoT lower than 15K and has 5 ticks or more are practically free. Even if you heal in the middle of it, you will get half of your mana back by the time its over, and already mitigated half of ts dmg. It is the least of your concern while being attacked anyway, unless it is the notorious triple DoTs of RDM. We dont even have to talk about other defensive cooldowns.

In case of long team fights that DoT can run through its whole duration, unless the DoT is really strong like RDM or SGE LB, it is usually just a nuisance and doesn't even matter as people can just take turn to heal as 15 seconds is long enough for a member to come back from death. This is why it is more important to CC and burst people to oblivion than applying weak DoT as it is also a huge opportunity cost for the attacker, unless that DoT has some debuffs, really strong, or you have no burst.

The only exception of this is when you can stack multiple DoTs on the same target and still has the ability to CC and burst them down, a feat that RDM can achieve solo. Long and weak DoTs are usually used as damage supplements, much like Monk during 1-6 combo but without potion interruption.
It would be a huge opportunity cost and a major threat in long team fights if there is no mana regen.