Calling it "Job Master" was a mistake on SE's part, you are not a master if you can't perform everything possible for the job.
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I have always thought that after obtaining master and getting the additional 500 jp's afterwards should award the player with something. I thought maxing out all of your jobs merited abilities would have been great cause not only did you master your job you took it further by getting another 500 points.
There's a point and time somewhere, though, that one just has to say "Enough is Enough". JP, just like Merits, is an endless sort of thing and we obviously can't be buffing our characters endlessly.
However, what you COULD do, is have the Potpourri goblin give you 2x (since it's 30k per 1, BUT you gain both simultaneously) Potpourri for each JP that way you have something to use the JP for.