Single Tank Encounter
Examples: Maexxna, Heigan, Loatheb, Sapphiron
These are encounter in which the only circumstances under which a OT will be required to tank is if the MT dies. Pure DPS gear is recomended.
Multiple Tank Encounter
Examples: Razuvious, Patchwerk, Gluth, Sartharion
These encounters require multiple tanks for the full duration(or close there to) of the encounter. OT's will take significant damage during the encounter and therefore full tank gear is required.
Optional Multiple Tank Encounter
Examples: Anub'Rhekan, Faerlina, Noth, Grobbulus, (Kel'Thuzad), Malygos, VoA
These encounters are most commonly done with a limited time OT, or an OT who will take very litle damage. In each case if sufficiently geared, the MT can take the adds which are normaly the OT's responsability, or the OT can be replaced by generic plate-dps who equips a shield for the situation(Malygos, VoA). Even if using an OT, argueably on all of these encounters the OT can wear pure DPS gear for the entire fight(although the advantages of this on Noth are questionable).
Limited Time Multiple Tank Encounter
Examples: (Anub'Rhekan), (Faerlina), Gothik, (Four Horsemen), Thaddius, (Kel'Thuzad)
These encounters are typically phased encounters or encounters with one-time-only adds. During phases without adds or once initial adds have been disposed of, the OT is free to DPS. This type of encounters usually has quite low damage on the OT and therefore the OT can either be replaced entirely, or the encounter can be OT'd in DPS gear.
Damage Mechanic Multiple Tank Encounter
Examples: Four Horsemen, Kel'Thuzad
These encounters have some mechanic which constitutes a threat to players or a burden on healers the more players are involved in an optional situation.
In the case of Four Horsemen, a tank who has finished with his job on Baron/Thane might decided to run in and start tanking or DPS'ing Zeliek. However, due to the nature of the fight, he now presents one more target for the healers to heal. Also, whilst moving away from Zeliek he may cause further links, increasing the chance of that his Holy Wrath will jump to multiple targets and insta-gib someone.
In Kel'Thuzad, whilst during Phase 2 no OT is required, each additional melee dps increases the number of targets for Frost Blast and increasing chane of Shadow Fissure in a highly crowded area.
What possible additional DPS the OT may provide in said situations is far outweighed by the additonal risks to the raid he constitutes. In such encounters, it is therefore better for the OT to simply stand off to the side than when not needed.
So my findings are that there is no situation, atleast in the current content, where the Anti-Tank gearset which Khalon suggests is actually useful. You are better off having two seperate sets of tank and dps gear.