If this is a FFXI reference, the it really wasn't at least for me, because for the most part you just used your highest level spell regardless of "weakness" since each spell was a bit stronger then the last, like thunder III doing more damage then blizzard III and stone IV doing more then both. Thunder III would not do more damage then stone IV on a crab, it just lessened the chance of doing partial damage (1/2, 1/4,1/8) There is an extremely short list of monsters that take static partial damage to elements, like elemental spirits, they do not take much to whatever element it is, and whatever element it is strong to. So if we use the little example of Stone IV, Thunder III and blizzard III, your strongest spell on an earth spirit would be blizzard III because of the static partial damage to the other 2 elements. It really wasn't that interesting because there was no increased damage for targeting a weakness, it was more of memorizing all the static partial damage stuff and knowing what to stay away from casting.
FFXIV however does have logic of FFXI's elemental wheel in the game, just hardly used. It is clear if you ever did a HW relic. For one turn in, you can do "astral-aspected materia, that is fire, wind, lightning, the 3 elements that made up the light skill chain in ffxi (the missing 4th be the holy element and we have no materia for that) and for Umbral-aspected it is ice, earth, water, same 3 elements for the darkness skill chain in ffxi, leaving out dark element.
But a real elemental wheel can't exist in this game since each job only has 1 or 2 elements, so it wouldn't be fair to lock out a job due to it. So that is likely why they left out the healing undead thing for 2.0+
Can't use Phoenix Down in battle though..
This as well, you simply did more damage with banish III or whatever, healing for damage never did much.