This is the defense that is always used when someone brings up horizontal progression and stat diversity and it really isn't true. For one, as stats get more complex, the less other people tend to care about your gear as long as it is level and job appropriate. But sure, someone will always come out with best cookie cutter builds for DPS Output, Healing Output and Damage Mitigation. But you can most certainly have different synergies in gear and on top of that you can also have bonus attributes like movement speed, resistance and a whole slew of other things that work out situationally or to optimize burst damage over sustained. You can't tell me that a +5% movement speed buff has no chance of saving a bad dodger in Titan EX, Ramuh EX, T6 can you? What if you are fighting something that does a lot of unavoidable AoE damage but isn't made any easier by high DPS, wouldn't you like the option of using a higher defense/HP set at the cost of DPS output? Hypothetical of course but what if you could stack enough resist petrify to never have to worry about Shriek, Voice or Petrifaction on T7 again. Or imagine if your tank found a piece of gear that makes it so if he gets hit for more than half his HP in a single attack it restores 25% of the damage from the attack. That sure would be nice for Death Sentence or Ravensbeak right?
I've clearly seen these kinds of things work out flawlessly in multiple MMOs. In XI on my BLM when soloing I optimized very specifically to ensure that I could 1-shot my mob or I would get dead real fast. For normal farming I pulled out a scythe to whack things with, then for endgame bosses it was all about Elemental Magic Skill to reduce resists. My Cleric in Aion has 5 sets of gear (HP, Magic Resistance, Block, Healing Boost/MP and Magic Boost) that I use in PvE depending on the fight (no gear swap mechanics here). Many Clerics would often fail at healing runs because they would die or wouldn't have the MP to handle it. In my DPS set I frequently clocked in the top 5 DPS of all players at open world fortress sieges gaining me top credit and making solo content go by absurdly fast. Man what I would pay for a DPS set on my WHM in FFXIV cause holy farming is amazing already.