You should check out the other earlier and longer thread about this. This argument has already been brought up there, discussed, and discredited by people who have experience with both jobs. I'm going to say up front that the people that actually discussed this have more experience than I do in tanking these instances. But I do have level 50 MRD and GLA and I have both PLD and WAR unlocked so I can follow the discussion and understand what's going on here.
What is being said by others is that PLD is not a replacement for a WHM. Parties with 2 WHMS don't need 2 WHMS because WAR is an mp sponge. They do it because it's safer and allows 2 raises instead of one. In both cases, you still have a WHM focusing on the tank and another focusing on the rest of the party. This is because PLD is more likely to be one shotted by boss abilities, so still needs attention.
Yes 1 WHM + 1 PLD + 1 extra DD is viable. But parties can and have already completed content just fine with 1 WAR + 1 WHM + 1 extra DD without any trouble because WAR has enough survivability and their DPS is just that much better than PLD. Things can be killed more quickly before it actually becomes an issue.
The gist of the argument is that on anything that matters, PLDs heals didn't make up for how dangerous a one shot ability from a boss is because they take time to cast and can be interrupted, and with their HP so low after a one-shot move, they are more likely to die, heals or not. WAR's hp makes a huge difference in survivability against abilities like these. Plus WAR has enmity tools that are just as good as, and some argue are better than, PLDs at the moment. That doesn't take away from the fact that you can tank as PLD. Of course you can. It's just easier and safer to tank as a WAR at the moment. At least before the changes. We'll have to see if that's changed now. But WAR's enmity tools and HP doesn't make it seem likely to me.
Here's the original thread if you want to check it out: http://forum.square-enix.com/ffxiv/t...aladin-Updates