Talking about an argument that needs to stand on better legs; assuming that an NPC is a particular job (this alone should be a red flag) because "he rages out". Not even gonna try and explain how magnificent of a roller coaster this idea is.
And another concept that seems to fly over your head as well as many others is that gameplay isn't so simple as you want to put it. Gameplay encompasses all aspects of a game that create an experience as a whole. This includes visual elements. Certain aesthetics can appear jarring to players and disrupt a sense of world or immersion that the rest of the game works to establish. Disrupting that sense of immersion is disrupting an element of the gameplay. If you cannot hold a very strong suspension of disbelief, seeing someone running around like they're attending a rave or night party, or seeing a chosen warrior tank a behemoth in a maid's outfit, then they would find themselves finding it hard to appreciate the game at its full value. I'm a bit of a prude and would prefer not to see such overly garish outfits, but y'all flex your creative muscles where the game lets you. That said, a line needs to be drawn and at the very least needs to be drawn to the very sets that define the aesthetic and nature of a job. The very sets that make a player feel like that job. Allowing other jobs to glamour those sets would make it feel disingenuous and take away a very specific feeling of playing that job.
Besides even that, there is an ever expanding library of equipment at a player's disposal for glamour purposes, and often will you see generic gear that was once tied to one job now recycled for another. I would even side for allowing generic gear for any job to be unrestricted without need of recycled sets. There shouldn't be some need to justify why there should be one set of gear per expansion being specific to a job to provide a sense of iconic imagery. You can have your huge pile of junk to mix and match, but leave that one thing alone for those few, that's all that should be to it.