A few quick points on this: (while I wait for the DF

)
Like with any job, you learn as you go. Some jobs you can have some prep/background for. But rarely do you know exactly how to do everything or can just jump in and know what you are doing at every step of the way. If your real world job you can, that is great, but in my experience that is not the norm for at the very least first time jobs (which is what these are in game), and rarely for well, any job.
Also, this game doesn't teach you any of the new job skills. It just gives them to you. No quests to train, no text bubbles. Just "here have a new skill!". So, in game, you are expected to learn the skills and how to use them "on the job" so to speak. And while it is true you are playing a character with a given job, in game context, you are doing it without outside support. These jobs are "ancient" jobs, hidden/gone from the world. You are mostly learning them on your own in game, without much support. As far as job rotation/groups, I agree that perhaps there should be more practice involved in game, but mainly through guildhests. People should be playing those over and over again for practicing groups and there should be more of them geared towards specific roles. These should probably be more widely used and tied to the main story to encourage them, but they aren't currently, so expecting people to have the skill acquired through such practice is naive: most people will learn again, "on the job" in dungeons.
Finally as far as dungeons go, in storyline, your job doesn't help you with mechanics or knowing what is required to win any given specific fight. In game you go into each dungeon "blind" for the "first time". So if you believe in story context, and adventure, then it is actually contrary to story immersion to go in already knowing the fight. It has nothing to do with "being prepared" and everything to do with story immersion/adventure.
If you don't care about that level of story immersion, that is perfectly fine and I don't think anyone would fault you for it, but don't expect others to feel the same way as you or try and force them into giving up the story immersion simply because you don't care to play that way.