Couldn't have said it better myself. It's time more people began to think outside of abyssea.
But think of other instances aswell, like dynamis for example, if your gonna go low-man (6-10 ppl) in dyna, you'll probably want someone with high defense to tank so that you can keep moving without having to wait for the mages to rest.
Abyssea, anything can tank, even mages, comparing anything with abyssea is pointless. Outside of abyssea, exp mobs are not worth comparing anything against, since normal exp mobs that people target, are generally fast, easy kills that yield high exp, so yeah... even thf could tank if they wanted to, but Monk tanking difficult mobs? Yeah... right.