ok let's be nice about this.

First off FFXI has way too many stats and bonuses that affect individual weaponskills and magic. We have bonuses that affect how fast a spell or ability goes off, bonuses that affect how fast we melee and how fast cooldowns are reduced by, we have bonuses that affect how accurate our spells, melee, and abilities are against mobs with high evasion, we have bonuses that affect how hard our spells and melee are against mobs with high defense, we have gear that affects individual spells, abilities and weaponskills, we have gear with magic defense bonuses when fighting against mage-types.

You would not see a summoner in FFXI fighting with Shiva while equipped with a fire staff since the elemental staves give a latent reduction in avatar perpetuation. Nor would you see a black mage solely casting in refresh and MP5 gear since they'd be missing out on bonuses that would increase the accuracy and potency of their nukes. And anyone fighting against a high level avatar-type boss without using gear that has elemental or magic protection bonuses will get their hides planted in the ground very quickly. Gear swapping in FFXI is a necessity to optimising the player's abilities and to increase survivability in individual battles.

I don't know a lot about FFXIV, but I assume the rules about spells and abilities are different and coincide more with other MMORPGs, like with World of Warcraft and Star Wars: The Old Republic, there is no "optimal" set for each ability. You simply work out what your soft caps are, aim for those in gear, and do adjustments to your gear to optimise it for ALL spells and abilities and not individual ones.