And for BRD to benefit (or even NIN to), there would have to be specific debuffs up for each attack. Which, again, would get cumbersome to have to manage. Besides, BRD is already reliant enough on a piercing resistance down rebuff that only DRG can provide. I'd rather not them be reliant on a "wind resistance down" debuff for higher DoT damage. NIN ninjutsu are only used once every 30 seconds unless you Kassatsu or Hide in the middle of a fight (the former has a CD and the latter not recommended at all), so again. BLM would be the only job that would consistently see any difference from specific elemental debuffs because of their Astral Fire rotation.
Again, better alternative would be a sweeping "magic resistance down" debuff, not individual "fire resistance down"/"water resistance down"/"thunder resistance down"/etc..