I would like to preview this by stating that I will be relating to the OP's questions in the last paragraph. But since Rog is trying to hijack the thread, I would like to respond to those posts first.
Proposal: In exp parties below level 74, a dedicated tank makes fights go smoother and more efficient to the point where it is required in the vast majority of cases.
A Damage Dealer is a class that is designed to deal damage without regard for defense or personal safety. They are equipped with low defense, low vit gear that focuses on damage dealing stats. In addition, they use job abilities that can cause even further lowered defense (i.e. berserk), not to mention eating food that boosts attack or accuracy and not defense.
A Tank is a class designed to build and hold hate, and reduce the damage taken. There are two kinds of tanks, those equipped for pure defense (defense/vit/eva gear, fish food, defender, etc) and those equipped for offense, relying on job traits and spells to reduce the amount of damage taken to manageable levels. Please note the word "manageable", it will be key later on.
As for anecdotal evidence, I have personally played three tank classes through most of the exp levels. When I leveled war in a semi-static, I used two war/nin as tanks fairly often, bouncing hate between them when shadows fell. This was back in 2004/2005, when sky war tp burns were just getting popular. My nin is all but retired at almost 70, and I recently picked up pld, where it is flying through the 50s.
When you go out to gain exp, the entire party comes down to two things: dealing a lot of damage, and surviving the fight. In fact, surviving is only required in order to kill quickly and constantly. In order to survive the fight, you can either reduce the damage taken, or heal it. If you heal the damage, that requires mp. And mp is a limited resource. The key, then, becomes reducing the damage you take by enough so that mp regen can keep up with heals and not demand resting or downtime.
A pure DD has very few ways of reducing incoming damage. They could sub nin, keeping utsusemi up to absorb hits. A samurai main (after lvl 35) can use seigan + third eye to great effect, but is less reliable than utsusemi. Unfortunately, both options require reduced damage in order to gain the defense. The few exceptions are jobs like thief and ranger, and to a lesser extent warrior, where using /nin can increase damage with dual weild.
The other issue with requiring your DD to have their own defenses, is hate control. I'm not sure how often anyone else has tried to tp burn before level 74, but just a single slow casting utsusemi makes life rough. Not knowing when you are going to be targeted makes timer and shadow management difficult at best. This is further compounded on mobs with double attack or multiple-hit weapon skills, since you have no quick utsusemi: ni to fall back on. As per my own experience, having two war/nin DDs trade hate worked exceptionally well ONLY because they both knew that it was their job to have the mob focus on them, and when to let the other war take it. It was about managing where the damage was going next, so we could be ready for it. Not going full out DD and expecting the shadows to take it.
Why does shadows or seigan make a big difference? A properly geared DD can take as much as double the damage as a "turtle" paladin. (A turtle paladin is one geared for pure defense, and I am not advocating it in this specific example.) Even if the paladin has hybrid DD gear, the DD can still take 50% more damage easily. This leads to spending large amounts of mp on cures, and can lead to downtime or even death if the healer isn't fast enough or doesn't have enough reserve mp.
Without the proper damage reduction, a healer can't keep up with damage taken. The vast majority of DDs under level 74 do not have the damage reduction required to sustain a party without frequent rests. Having a dedicated tank reduces damage taken to a level that allows the healer to maintain mp indefinitely, assuming proper mp regen and camp location. It also allows the DD to focus on dealing damage, often more than making up for the amount of damage lost switching to a tank.
Having mp regen roughly equal mp expenditure is the ideal. After all, there's no point in gaining more mp than you use. If that is the case, the tank can take a little more damage in order to deal a little more damage. Some turtle pieces can be swapped for damage pieces. Note: nin turtles with evasion, pld turtles with defense and vit, so I'm not only assuming one or the other.
That leads to the point everyone always says: it depends. Sometimes, you want to go full-out attack on paladin, since you have high base defense and vit, not to mention a shield that blocks a ton of damage. But sometimes, you might want to swap out a few pieces for some extra damage reduction. It's a juggling act, trying to deal the most damage possible (for extra hate, and to kill it faster) while making sure you aren't taking too much damage.
Doing pugils at lvl 22? Fish kabob all the way. Reducing damage by 15-20% made it easier to hold hate, and made it so book refresh meant we never had to rest. Mandies? Meat for sure. They already hit like unarmed black mages, might as well kill them as fast as possible.
The whole point is that turtling is rarely the answer. But neither is going full attack. The best tanks find the sweet spot.
Enfeebling from a tank point of view is pretty simple. If you have a ninja tank, wait a couple of battles to see if they use Hojo or Jubaku, their version of Slow and Paralyze, respectively. Most low level ninjas don't, since the early versions suck, but they get hate from it so let them cast if they are going to. If they aren't casting Hojo, slow is a priority. Slow makes it easier to cast between mob swings so they don't get interrupted. It is actually the same with paladins and cures, slow makes things easier. Paralyze does the same kind of thing, but it is random so you can't actually count on it. Blind doesn't matter too much, but ninjas might make good use out of it before lvl 37. Even still, the hate from casting blind can make it dangerous for the rdm.
RDM75/BLM59/WHM37 Maat Masher 1/1 ^^
WAR75/NIN64/MNK46/DRK62/SAM37/THF37 80 Paladin 80 Mage