Damage over time abilities (e.g. Fracture, Touch of Death, Venomous Bite) yield superior DPS, so it's ideal to apply every available DoT to every enemy (unless you're using AoE), at least once at the start of each pull. Marking targets and using only single-target combos yields inferior damage.
Marking targets has nothing to do about damage, and everything about making tanking and healing easier. Just because they're marked doesn't mean you can't AoE or put up DoT's on secondary targets, it's used to mark the kill order, because using AoE abilities is resource intensive and not nearly as effective as single target killing in regards to tanking and healing.

It's much easier to heal when the amount of damage coming in lowers over time, and when 1 of 4 mobs dies the incoming damage drops by 25%. It's also easier to tank and heal when everyone is attacking the same target rather than having every mob attacking a different party member. Even great tanks can't hold threat on mobs if every DPS is attacking a different target. The AoE threat tools available for them just don't cut it when a DPS is going full out on a mob.

Unless my fundamental understanding of numbers is wrong, marking and focusing one at a time is worse DPS.
Again, it's not about DPS, it's about party cohesion and effectiveness. I don't care how much DPS you're putting out if peple are dying or it's a hectic pull every time because you can't hold threat due to everyone atacking different targets.

Marking the mob doesn't mean don't AoE or put up DoT's it means "kill them in this order."

I've even explained how the maximizing of DoTs is superior damage and makes for a faster, easier run
I'll say this one more time: marking targets does not stop people from using DoT's or AoE Stop being lazy, when everyone knows which mob to focus on, the groups runs smoother. And I have my doubts that your method yields runs that are any faster than one from a tank that marks a kill order. I'd actually believe the opposite, because when mobs are running everywhere and you can't focus on one target to kill and the healer is having to go into overdrive to keep everyone from dying, things go slower.