I've wanted this ever since Glamours were released, but 4.0, alongside recent event rewards and mog station items, have really begun to deteriorate the argument against letting anyone glamour any piece of gear.
So the biggest issue against this is being able to identify other players by what they wear. Yet, we have event reward, mog station, and even base game equipment that 1.) anyone can wear and/or 2.) don't suit every jobs' aesthetic. So already we have issues with identifiability. The easy example is bikini tanks or goofy full-body outfits like chocobo and pig outfits, but looking recently, we now have the Shisui sets which are all skimpy recolors. and the Gubal sets, which are all the same but different colors. I'm certainly not saying anything is wrong with these items, but they break the 'rule' that equipment is going to have some sort of aesthetic pattern based on role. If the base game is going to break that rule (in addition to event gear and mog station gear), why have the rule at all? Let healers wear some armor, or let Dragoons wear a robe if they want.
Second, we have role gear that doesn't satisfy the jobs that fall under that role. A prime example is Striking gear, which include Monk, Samurai, and Ninja pre-50. I think its fair to say that at least some Samurai players want to wear the traditional samurai-themed armor that Tanks have (Genta and Arhat for example). Naturally, that doesn't fit well aesthetically for Monk, so instead Samurai are stuck with whatever fits with Monks.
A similar issue is Aiming gear. Sometimes they're reskinned Striking gear, and other times Aiming jobs get reskinned Casting gear. Make no mistake, I understand the need for reskins: modelling/texturing/etc is a lot of work, but it still creates an issue of Aiming gear not really having its own identity. Sometimes I look like a caster, sometimes I look like a Monk. Sure, I can glamour something else, but that's new content and it breaks the pattern. 99% of the time, healers and tanks always get gear that look like healing and tanking gear; they never have the issue of new gear releasing and it not fitting their aesthetic. For Bard and Machinist, its a coin toss on whether or not new gear is going to put me in a dress.
If the rules are going to applied at random, why have the rules? The problems associated with no restrictions on glamour are already present, so who is going to have a problem with this who doesn't already? This change would let players experiment with hundreds more equipment pieces in their glamour, which many players enjoy doing. It makes gear more valuable, which is more motivation to craft and run dungeons. We can even already sort of do this with the gear preview window! Just let us do it outright.