So...about Arcanist

[Spoekes]

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 would guess every class/job is limited to one role, for reasons of balancing and simplicity.
You'd still need a way to choose wether heal or dps when you queue up.
Short put, they would need to rework DF ,which might not be as trivial as one might think and they might think the time is better spent somewhere else.

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.

Whats your point here? Can't see what they're doing?
Last time I checked healers contributing some DPS while keeping the party alive are a good thing.