From Psiven of the guild Prodigy on the server Dalvengyr. He's a tankadin who specs back between Prot and Holy as needed by his guild. I'm putting it here for future reference.
His thoughts on Prot Paladins beyond Gruul's Lair:
"Beyond Gruul's, think about it, and have your GM/officers think about it.
Magtheridon: Who better to pull 3-packs of trash with mass deaggro effects? Why swap out a tank at the boss when you need 4-5 to complete the encounter?
Hydross: Can you think of any other situation where you'd really want to bring 4 prot warriors? Why use a feral druid on adds when a paladin takes 10% less spell damage and has access to epic resist gear?
Lurker: Who can frontload threat on Guardians that need to be burned down more efficiently than Avenging Wrath?
Leotheras: Would you rather have tanks chasing him around during whirlwind and slowly backing back into position, or have him run straight back at full speed to a paladin's Avenger's Shield? Does an HP difference matter when he can never crush?
Karathress: Who else can feasibly tank both the hunter and his pet to be killed first?
Tidewalker: Warriors and druids can't consecrate, no matter how hard they try. Alternatively, a paladin will basically never be crushed.
Lady Vashj: You only need 2 full time tanks, but nullifying one of your best healers or ranged DPS to handle Cores probably isn't a good idea. If it's a choice for the 2 tank spots to raid that night, who else can pick up elites from 30yd, get Vashj to shoot them without needing BoF, provide a BoF of their own, and guarantee uncrushability in the face of her extremely fast attacks (dual-wield hitting for about 5k main hand)?
Void Reaver: Who can pack more MT+OT threat than a paladin with infinite mana?
Al'ar: Why not just tank all those pesky adds instead of trying to make them explode?
Solarian: Mages love when consecrate is ticking for 200.
Kael'thas: 5 weapons locked down right away as needed. AoE to taste, add salt before serving."
And now his thoughts on using a Tankadin in Mt. Hyjal and Black Temple
There's no strict reason you can't use a pally MT on:
Every wave of Hyjal trash
Illidan (unless you have 3 prot warriors)
There's a good reason to use a pally tank outside of MTing on:
It's just a bad idea for a paladin to MT:
Essence of Desire/Anger
Given equal choice with a warrior for the spot, there's a good reason for a paladin to MT:
Most of Hyjal