It really isn't though. It just becomes "Spam fire at Ifrit" instead. Toss in a Blizzard spell to regen MP and then its back to fireballs again.
The beauty of having elemental vulnerabilities is that each enemy becomes its own encounter. Instead of spamming the same boring combination attacks. Every. Single. Time. On. Every. Single. Mob. you can actually be smart about it and mix things up during combat. Some of the BEST fights in FFXI were due to elemental resistance or weakness, or using the right skills to defeat the opponent.
And that's what this game is REALLY missing. Each fight needs to feel different, but they don't. Random mob runs up, you hit it with the same tired combo you've used on EVERY mob til now. Doesn't matter if you're a Paladin, or a White Mage, a Black Mage or a Summoner. Every single job does the same thing they've always done on every single opponent. Booooring.
Bosses mix things up only in how you dodge their attacks. How you assault them makes no difference in the long run, with the few minor exceptions being mob that shouldn't be attacked at all. Elemental weaknesses might not be the ultimate cure for this monotony, but it would sure go a long way to livening up battles.
Except, you're still limited. Black Mage is limited by their game mechanics, so they basically use Fireballs for Astral, regen MP with Umbral. It's so robotic, I daresay it couldn't be much more limiting. Summoner is basically limited to Garuda, since both Ifrit and Titan suck for any real content. Monk is HIGHLY limited, with most of their skills unusable until they're in the correct stance, so it follows a very predictable pattern. Would you like more examples?
The point is the game shoehorns everyone into neat little cubicles so that everyone plays like everyone else. Those who don't will invariably suck at their job. Every job has a rotation, whether it's Dragoon, Bard, Black Mage or Warrior. So every job is 'limited' in the sense that everyone has to play the same way, or fail at their job.