There's plenty of people who can play their class even though they're only/mostly farming fates. At the same time, there's plenty of people doing dungeons and still being awful at their class.

People will learn how to play only if they want to learn, it's simple as that, I'm afraid. Not much can be done about it.

Of course, fighting some bosses and being in certain situations in dungeons is an useful experience for learning to be better at your job, no questions 'bout that.