As someone who loves OTing in WoD, I just wanted to point out the things that tanks can do besides be the MT. Now that I'm working on my BLM Zeta and have to queue as DPS, I've been finding more and more tanks in WoD who just fight over the MT position and ignore the rest of the fight. It's very irritating and slows down the raid. So, here are what the OTs could (should, in my opinion) do during each boss fight of WoD:
The first boss has a large number of instakill mechanics. If the boss turns, DPS and healers die very fast from Double Vision and Stare, especially if he completely flips to face the other way. The only times I've wiped here are when the tank died to an instakill turning the boss and half the raid was dead before another tank got around to both provoking and repositioning the boss. As a result, I feel like the "correct" way to deal with the boss is to have 2-3 tanks "MT" it. Stack tight on top of each other and get your aggro as high as you can. It's like fighting over aggro, except it isn't fighting because it doesn't matter who has the aggro, only that both of you are the two highest on the aggro chart. If one of the tanks dies, the other tank is top aggro and nothing changes. When the dead tank is revived, they can provoke the boss to get back to sharing the top aggro spot with the other tank in case another death occurs. I've never wiped this way, though I have pissed off other tanks who didn't know why I was "fighting them for aggro." Also, obviously (I hope...), if you're not the tank that currently has top aggro on the boss, pick up the adds. If you do that while still standing right in front of the boss stacked with the MT, your DPS can AoE them all at once easily.
During Five-Headed Dragon, there are a bunch of mechanics to take care of. A single OT can take care of nearly all of them by herself allowing DPS to focus on, well, DPSing. As a tank, an OT can grab the fire drakes and soak all three fireballs (especially PLDs, who can Stoneskin themselves after each fireball). (On a sidenote, there's nothing more annoying than grabbing a fire drake and, before the first fireball is even fired, having a DPS or healer run in, grab the tether, not pass it on, and die. Seriously, see a tank with the tether? Let them keep it :P) When the poison AoE drops, tanks are best suited for standing in the poison and keeping aggro on the slimes so they die close enough to grant the people standing in the poison the poison immunity. Without a tank there, many times a DPS across the map pulls the slime and the people in the poison don't get the poison immunity, soaking way more damage before the slime goes away. OTs can also watch for the purple lightning breath indicator and run in to help soak, popping CDs to minimize the damage caused by the split attack. PLDs can even Cover the target, pop Sentinel, and soak the whole thing on their own. Of course, this doesn't help when people run away from it, but still. I find dealing with these mechanics to be far more useful than the little DPS that a tank provides, though nothing stops you from DPSing between mechanics.
For Cerberus, it should be obvious what the OTs should be doing - tanking the Wolfsbanes. Just be careful when tanking 1-2 Wolfsbanes and picking up another - the Wolfsbanes have a push attack. They seem to love to use it when you are between them and the slobber, pushing you into the slobber and instakilling you. To prevent this, never let yourself stand directly between a Wolfsbane and the slobber (or, if you're a PLD, pop Tempered Will first). Don't fight over the Wolfsbanes. If you're the only tank going for the Wolfsbanes, by all means take all three. If the other OT is good and also trying to help tank the Wolfsbanes, split them up. If they get aggro on the first one, just wait for the second and grab it rather than trying to peel the first off of the other OT. (This should all go without saying, but in my experience, nearly every time the other OT actually goes for the Wolfsbanes, they go for the one I'm tanking and ignore the newly spawned ones. If this happens, just let them peel the first Wolfsbane off of you and focus on the next two.) Also, even if you want to only send a single group's worth of DPS inside, there's never a reason for a tank to go inside Cerberus. The Unknowns deal laughable damage and are not the threat. Stay outside and tank the Wolfsbanes - it is much more important.
For Cloud of Darkness, I feel that OTing is much the same as with Five Headed Dragon. Focus on mechanics and not DPS. You should have a sense by now of when Meteors come - when they're due, get to the other side of the arena far from the boss and get in a meteor circle asap when they spawn. If you wait too long to get into the middle of the circle, a DPS or healer will run there too and unnecessarily take the damage. When clouds start spawning, be ready for them and focus on slowing them down. When lurkers spawn, run around using your basic attack and OGCD attacks to interrupt them. That allows the DPS to spend more time burning the boss. And, be ready with LB - if the group is sub-ideal and lets Cloud of Darkness eat 3+ stacks worth of clouds, pop the LB when she speaks after the cloud phase.
During no part of WoD is a tank's DPS significant enough to speed up a fight noticably. However, during many parts, if the tank deals with mechanics, DPS can focus on DPSing. When tanks aren't dealing with these mechanics, it's not the bad DPS who deal with them. Usually, it's the good DPS who know what they're doing, and they generally can't DPS while dealing with the mechanics. Also, if tanks are soaking damage from mechanics like meteors, lightning breath, fire drakes, and poison puddles, DPS are less likely to die.
Also, during the first two trash fights, please pick up adds if you do not have aggro on the central enemy (Two Headed Dragon, Scylla, Xande). Then, take the adds to the middle so DPS can AoE the big enemy and the adds all at once. When dealing with the very first trash pull with the three Chimeras, pull them away from the healers and ranged DPS. Take them either to the center, or, better, the far right or left of the arena. Healers and ranged DPS will stack right at the entrance, and hitting them with constant paralysis (which has a much smaller range than caster spells/BRD attacks) really doesn't help clear it quickly.
/psa