I take umbridge with the word "EVER" in this sentence, Blu/Thf, Blu/Rdm, Blu/War and even Blu/Whm and later Sch curing with a staff were all reasonably frequent party setups. (The last one was a bit more rare.) If you don't believe me, go look at all the old forum posts from the years after Blu came out, I'd almost wager Blu/Thf was more common than Blu/Nin, at least in the Lv30~72 range, and again on Dark Ixion and Odin for Frightful Roar and Sneak Attack Cannonball. I main healed many a party when we couldn't find a healer, and occasionally I would /War and tank with Provoke and Cocoon.
Thf also would sub War somewhat frequently, because Berserk was better DPS if you didn't need Utsusemi shadows, though it was definitely less than 50% of the time, admittedly.
Dnc and Nin are the only jobs that I can think of that exclusively Dual Wielded, well, outside of the times Nin was using Earth Staff to tank or kite-tank, which did happen sometimes.
Any situation where dual clubs comes out to more cure potency II, or macc than your current staff options it makes sense for a Whm to naturally be able to equip 2 weapons, as I said, put a giant slow penalty on Dual Wield 0 and you don't have to worry about it ever being a DD trait lol as it would decrease your melee DPS.
While I agree Dual Wield was intended to be a DD trait, so what, Ninja was intended to be a DPS job as they're so fond of saying, but for a good 7 years or so they were our best tank job, and that was really neat, things can change if it makes things more interesting or fun at the end of the day.
Also, there's a bit of hilarity here, if you care about developer intentions, Whm WAS meant to melee with the party, don't believe me? Go watch any of the old dev-released videos of their party play. Not that I really think this matters, but it's funny, Hexa Strike is Whm only, all Whm bones parties were a thing, and to a surprisingly large extent, they did intend Whm to melee, heck Whm relic is built around meleeing, though the Mythic is certainly useful without it.