If people want to be compulsive they can.
This seems a bit inconsistent, though--remember discussions about Gained JP Up, where you decided that even if we reduced it to, for example, a 10% JP boost, that it still wasn't a good idea to include it, because some people would be compulsive and use it?
(Or they would just level up a Mime if they care that much. Which is fine by me!)
I don't have a problem with people levelling in Mime. Here's the thing though: they lose something by levelling in Mime; the whole time they're in Mime, they aren't gaining JP. It's a trade-off. (On the opposite end of the spectrum, Chemist hands you fantastic abilities, and crap stat growth; also a trade-off).
Now compare this to, say, 40 MA growth calculator. Plenty of people spend ages grinding in Calc for Mathskill (or even Non-Charge). Calc is already giving you the best abilities in the game. Now you're suggesting also giving them among the best stat-growths in the game. Where's the trade-off?
And in a broader scope of the mage half of the job tree...what does a mage really sacrifice by never leaving that half of the job tree? A couple of higher-HP, lower-MP carriers? Maybe two skillsets they'd want as a mage (Draw Out and Dance) while maintaining easy access to...9 good skillsets? At most a minor tradeoff.
(And as far as PA growths go...there's all of two generic skillsets that actually use PA to a significant degree--Punch Art and Jump. You'll notice Monk has less impressive PA growth than other classes--again, a tradeoff. Lancer...doesn't have a tradeoff; I might be worried about Lancer if people started complaining they were overpowered >_>).