So...about Arcanist

[Nyalia]

What about a conjurer signing as healer for the harder content? Or any level 50, with the way everyone likes to speed run. The conjurer would have zero cooldowns to use on a pull. That's no different from an arcanist signing as healer, and its currently possible to do.

I disagree with this statement. ACNs have no healing skills beyond Physick and Resurrection, even at 50. CNJs have plenty of healing skills beyond Cure - Esuna, Raise, Medica, Cure 2, aspected Stoneskin, Cure 3, Medica 2. The only healing skills (not counting CDs) a CNJ would lose are Regen and Benediction. While those two are very useful, you can't say that a CNJ at 50 is "the same" as an ACN at 50 as far as healing ability goes. Plus, there's the gear thing - ACNs at 50 cannot wear endgame healer gear. CNJs at 50 can. Gear does matter.

Before level 30 THM/ACN can easily heal dungeons, so I too find it odd that they can't queue as healer for said dungeons.
Trolling higher level content is easily prevented by only having the option to sign as healer for <30 content.

I'd guess the reason it's prevented is simply consistency, especially around level 30. Allowing an SCH-to-be to queue as a healer at 29 brings up the weirdness of leveling to 30 while in the dungeon and how the game handles it. And then what about ACNs who queue at 29 and then level to 30 before their duty pops? Sure, there are solutions to these things, but the more complex the situation, the more coding is required. Personally, I don't think the benefit outweighs the cost. And, again, SCHs have fairies that make them just as good as CNJs at healing low level content. Without the fairy, they're much worse, but that's why they have the fairy. The fairy heals more than the SCH at low levels, so an ACN would have less than half the healing power of an SCH.

Which people already do anyway. I suspect over half of the DF scholars spend more time dpsing then healing, since they are well aware you can't see what they are doing.
And plenty of white mages just don't care.

Except while they are DPSing, their fairy is healing. So, they are fulfilling the healer role, even if they are dealing damage at the same time. This would not be true for an int-spec ACN. Note that Cleric Stance does not work well in reverse as it gives a healing potency penalty while active.