I dunno. I like it in theory, but in actual practice that would totally screw over a few classes that make heavy use of certain elemental attacks with no real choice on switching up spells (like BLM) . It seems unfairly stacked against them since while every class uses a few different damage types the majority of them all focus on one certain type of damage. unless you want to add things like Blunt resistant mobs for MNKs, piercing resistant for DRGs/BRDs, threat resistant mobs for tanks, ect.
I mean, just imagine an endboss like Twintania randomly being resistant against Fire and being weak against piercing, which would be a "realistic" scenario. That'd basically kick out all BLM and MNK mains from Turn 5, and DRGs/BRD would be stacked. Even if BLMs could build Magic pen or whatever, ithat'd mean taking away from their current primary/damage stats to do so, so they'd never catch a class able to take advantage of the boss's weakness. That is just a general example, imagine getting kicked from a 4 man DF after waiting an hour in queue just because it had mobs/bosses with heavy resistance to your damage type. This type of thing happened a lot in FF11 and several older MMOs like EQ and it led to certain classes being much less desirable and wanted for all kinds of content. Even if it makes the fights more boring in some ways I much prefer this game's point of allowing skilled people to play the class they want without being shunned (more or less).
Not calling you out personally, but I think a lot of people on GAF underestimate the amount of people wiping on the game's hardest content now. Even in this thread, some of the people struggling to find statics/groups capable of clearing stuff can be heartbreaking (and the amount of people calling the content easy while wiping to it on any given attempt is...weird?). I'd like more gear progression variety to be sure, but I think for the time the game's been out, the content's difficulty has been fine. We just need more content period, which is coming.
There's also the problem of there not realistically being enough space to hold full sets of resist gear on top of what we already have to level crafts and various jobs, plus everything else like vanity and holiday stuff. I have more room than some, but still...