the need for 3rd tank class

[Allyrion]

Because certain jobs attract certain players. There were people playing NIN tank in FF11 who would otherwise never have touched a tank job (funny because NIN wasn't intended to be a tank, but that's beside the point). Some players choose their class by role, but many (probably the majority) do not. They pick what looks cool and interesting to them, and whatever that job does, they'll play it. If Dragoon armor were simple grey armor with no spikes, I doubt we'd even have "loldrg" to joke about.

Exactly. People have job identities more than they have role identities.
Just cause you play one dps doesn't mean you'd play another. Same for healer.
If you add more tank jobs, there will definitely be people who don't like the current tanks that would like that.

Not to mention that simply adding a new class will always lead people picking it up just to try out the new thing.
It may be temporary, but at least more people will try tanking cause of it and the stigma will be lessened.