It's really poorly implemented at the moment, and probably will remain so without some significant design philosophy changes.

Elemental resistance tends to work on a sliding scale: Either it's so weak that it's irrelevant and can be survived anyway, and thus people generally won't bother - or it reaches a point where it's necessary to survive things, in which case you put extra weight on players to have an entire separate set or pieces of gear just to deal with it.

Both options are valid; XIV's philosophy seems to not want players to need to carry around multiple sets of gear. But it still has the mechanics, for some reason, so it falls into the former camp, where it just sorta looks like a sloppy loose end.