Even in the single-player games there are any number of jobs that had self-healing and life-leeching, any number of jobs that focused on evasion for multiple turns or negated damage entirely.
And even among them, very few had gameplay loops centered around rapidly spending their own surplus health as a resource for their unique skills. Black Mage didn't even do that, and it could replenish its own MP with Osmose.
So if you didn't have your pocket White Mage, how was your Dark Knight supposed to recharge his Darkside? Or in the examples you listed, his buffs, once they ran out and he was left without all of the health from damage he took to stack them in the first place? It's not like he could just mitigate damage to recharge them -- the effects specifically state how much health he has to lose to charge them in the first place.
So once he shoots his load, how does he reload? Especially if you expect him to pull double-duty of getting beat on to protect any remaining back-row squishies?
In Bravely? He didn't. He needed external healing. If you wanted to go with a Barrier Tank example, he could have used some exploitable substitute to take the damage for him and charge his effects, but that wasn't an option.
In games where he had Blood Weapon or some variant of Drain? He used those, which formed a self-sufficient gameplay loop. Yes he still needed a healer if enemy damage was high, but he didn't just Darkside to death.