You mean "sub white mage because the game lacks dedicated healers so you can be shackled to healing and spamming buffs", "sub black mage and get laughed out of groups because your nukes are still subpar and are 'less useful' to the group because you're not focusing on healing", and "sub WAR/NIN/DNC and be kicked off the game from the sheer power of the alliance's collective laughter at the idea of you wanting to do something other than heal/buff"? RDM is not the best example for what you're trying to argue.
If you argue abilities, then you're conveniently forgetting that this is done for the sake of balance between jobs, in tandem with traits that enhance said abilities for their native classes.This game presently offers you no choice in gearing. There is a best in each tier. There's no choice in stats, there's a best for that too. There's no real choice, for paladin at least, in what abilities you equip and it's not because there can't be, it's because they choices they offer are silly.
The abilities don't have to be "useful" 100% of the time, as the core of jour job is defined by native class abilities and the five job abilities you get. Cross-class abilities are for additional utility and flavor, but are not and should not be job-defining. From one paladin to another, my bars currently have Cure, Raise, Foresight, Mercy Stroke, Fracture; outside of Foresight, they all don't get used 100% of the time, but that is fine.
If you argue gearing, you're basing that entirely on wanting to be a unique snowflake with no practical gain to gameplay on a larger level. Keeping stat gains and job/class scaling on a tight leash is for the sake of balanced gameplay, and it'll be a cold day in hell before I support the alternative.
If the choice is between "balanced classes with a very limited selection of cross class abilities that 'place themselves' while being more for flavor", and "balance clusterf*ck where people might think they are special snowflakes but there is in reality one sole good way to do things and a trillion under-performing set ups that get laughed out of groups that do the content that matters", I'm going to pick the former every time.We may look different, but we're all ascending to be precisely the same thing on each job. Worse, there's not even any opportunity to get it wrong. If something is higher tier, you equip it, and the sub-job abilities practically place themselves.
-------------------
Now, if you wanted something a little more cosmetic, I may support that. Going off the minor glyphs from WoW that modified abilities cosmetically without changing gameplay, maybe even adding silly flavor abilities or modifications to spells and special effects. I'd love a Glyph of Mercy Stroke that changes the animation to simulate the weapon type you're using so that instead of a giant axe rising from the ground a giant sword raises from the ground (would sort of ressemble Night Sword from FF Tactics, so you can even tie an FF reference to it!).