I agree that jargon is risky. If your mages have 0 jargony terms, nobody will lose interest in your work because of it. No reader will be going "I just can't believe there isn't any jargon here!" even though, sure, of course there eventually would be. You can look at the chatroom of any MMO to see that everything gets shortened and nicknamed to things that are easier to say and talk about, and sometimes those nicknames are silly. It's realistic to have jargon!
But "realistic" isn't always the same thing as "believable." And the thing is that, yeah, if you do jargon really well, it's extremely rewarding! It can make the unimaginable feel more real, because here's people treating it the same way they'd treat real things. But if you do it poorly, it's distracting. And that's not just a matter of using it well, or teaching it to the reader slowly, but also a matter of just...... inventing words and phrases that seem reasonable. Like, if you make your own curse words and they sound dumb, it doesn't matter how gradually you introduce your alternate profanity system, we'll always be thrown off by how that's not a sound that any human can imagine shouting when angry.
create casual jargon> do it well = richer world
create casual jargon> do it poorly = distraction or loss of belief in world
don't create casual jargon = no one will notice
So it's kind of something I think is worth into if you really enjoy playing with these kinds of ideas, but otherwise it's not required. Ditto for inventing a language -- if you like inventing languages and it's fun, go for it! But it doesn't really add much if your heart's not in it, and nobody's gonna be mad that you didn't invent your own languages for your fictional world.