I think Tanaka's goal was to create interesting new classes using abilities from classic FF classes and mix/matching them. If the game hadn't been such a flop it would have been interesting to see where he took them. But, since it did fail so hard, Yoshi-P was brought in and said
"You call this a Final Fantasy? There must be moar FF staples! moar!"
And so they had to shoehorn the classes into jobs. The class with all the lore about elementals and harnessing their power becomes White Mage (with Aero and Stone because...erm? elementals? but not the icky fire or ice elementals). The class with the lore about the giving and taking of life and the balance of life forces becomes Black Mage because they had the strongest nuke at the time.
I actually levelled all but a couple of jobs in XI to at least 75 (the cap when I levelled them) and the ones I didn't were Corsair, Puppetmaster and Red Mage. I honestly didn't feel it was that hard to get parties. Of course, BRD got insta-party without even flagging up I would get invites as soon as I logged in, but the other classes? My DRK rarely waited noticably longer than my PLD, SCH, WHM or NIN.
BST I obviously rarely got parties on, but Duo'ing was fun. BLU got a lot of invites especially when I used to /THF in Wajaom/Bhaflau.
Even if there were discrepancies in wait time for you (I don't doubt there were, it just wasn't the case for me), in the end, wouldn't you say it was worth it for the differences between the jobs?
There was no job that played quite like THF (setting up SATAs, controlling hate), no job that played quite like NIN (balancing shadows, constantly scrabbling to stay top of the enmity table) or DRK (Last Resort, Souleater, Guillotine, manic laughter, insults from the WHM) or BLU (Self-SC'ing, MB'ing, masochistic spell learning) or SCH (AOE every buff, wake me up when they wear off).
I could literally give examples for every class where they had a playstyle-defining mechanic that none of the others had. What's a little queue time compared to that?