With Staff being an exception to RDM, I'd almost say the reason should be more that if you have an offensive spell of the corresponding element, you should be able to go for that particular path. That would put BLU on for all 8, PLD for Light, DRK for 7/8 (no Light), and NIN for at least 6/8 (Unsure on the alignment of some of their new spells). Some jobs might not use these much, if ever, but having the option should be there given the intended purpose of boosting the spells.