On the subject of elemental resistance for players, I think you'd be better off buffing up duration and potency of Resistance Ward Potions than playing with resistance gear. Ultimately, it's less problematic in the long term and more healthy for economics.

On exploiting enemy elemental vulnerabilities, it's a great mechanic....in Pokémon. Unless a game is designed to utilize such a mechanic it's just impossibly difficult to balance it out. FFXIV, for better or worse, just doesn't have vulnerabilities to such an extreme that one is useless/Mandatory against an enemy for the class balance reasons mentioned here.