Yeah, I regret the title a little bit, but I was referring to things that have lower level than 119, including content that inflates the monster's stats without actually affecting their level (like High Tier Battlefields).
Your thf friend tanking actual old stuff is fine and sensible. It doesn't make sense to bring a tank when a tank is not needed. A PLD that can't get hit (because his evasion is too high for some older NM) is pretty useless--mch of our TP comes shield blocks).
If a friend needs something old dead, say an abyssea NM and actually needs help, I'll go on blu or something and dd-tank as much as the mob wants to hit me.
I'm talking about a level of content where enmity is fractured to the point that it's not working (like old content) but still dangerous enough that you need the tank to be able to reliably tank.
Edit: And there's a ton of more-relevant things that many jobs can tank, which I don't mind. There are also things that need a dedicated tank and enmity is fracturing. Or just cases where you bring a dedicated tank and he has to struggle because of deliberate design.