Resistances are more relevant right now than people think. Someone tested it pretty well and found resistance stacking has a sizable effect on successfully protecting someone from a confirmed elemental attack.
The issues, though, are obvious. Needing a gearset for each element isn't efficient. It's generally safer after a while to just burn stuff down. Attacks with enough damage WORTH mitigating generally appear unaspected, unless you're just REALLY into pony farming with some terrible luck on your side to justify resisting those high damage blows. And it can still get expensive to max out resistances in crafted armor.
I'd imagine piling on resistances goes hand in hand with piling on VIT melded accessories in new content. Useful for learning, shaved off bit by bit once it's figured out, removed to the minimum (zero, in resistances' case) once it's mastered.