Elemental Resistance: Why Even Have It?

[Garlyle]

Why though? What's wrong with an elemental wheel? How do its negatives outweigh the positives?

In analysis I'm not even sure there's a positive.

If you want players to target an elemental weakness, you end up with
1. Characters are now forced to carry more gear to be 'properly equipped' for encounters, forcing a lot more farming. (There is an argument though that this is a positive for things like reducing content obsoletion, but this is distinctly not the philosophy XIV takes)
2. Classes spontaneously have various tools restricted from them in fights simply because they're not the right element.
3. Classes that do play the elemental game and avoid 2... now have six functionally identical spells that aren't actually adding any gameplay because you're using the same one over and over and ignoring the others, creating unnecessarily clogged skill lists.

It's not even a 'smart' thing. It doesn't take intelligence to pick which spell to use, just foreknowledge of which one's most effective. The only time it might get interesting is bosses which rely on changing elements, which has heavy impact on other classes in the process.