Dev Team, you screwed up.

[Valdegarde]

Explain to me in which content a Summoner is tanking in place of a tank and a Red Mage is the healer.

Ramuh Ex and every damn Alliance I've ever been in, respectively. Also I saved one of my early Halicarnassus savage clears by alternating getting people up and keeping the tank afloat.

Monks have Riddle/Fist of Earth too. Classes have lots of options actually, you get to see some of them in things like Palace of the Dead. They used to have even more before skill pruning and consolidation. Featherfoot/dark dance = Toad Oil.

[Ilyrian]

Ramuh Ex and every damn Alliance I've ever been in, respectively. Also I saved one of my early Halicarnassus savage clears by alternating getting people up and keeping the tank afloat.

Monks have Riddle/Fist of Earth too. Classes have lots of options actually, you get to see some of them in things like Palace of the Dead. They used to have even more before skill pruning and consolidation. Featherfoot/dark dance = Toad Oil.

If this were made more official - i.e Summoners could be legit tanks and RDMs legit healers, then I'm sure the game could make BLUs a proper jack of all trades like RDMs really should be.
Currently - I don't think that's the design philosophy of the game. Given that we have dedicated tanks and healers. What do we do with them? Why bring a tank if a summoner can do it and do more damage?

I'm not against it - it would be more interesting... a nightmare to balance obviously, but I'm sure no one really cares about balance...