The problem, as someone pointed out before is even if they drop the GCD or you get tons of gear that drops the GCD, you'll still run out of TP eventually and then it's thumbletwindling time again, actually for longer. They do need something to regen TP, maybe pots? I think there's only potions and ether but nothing for TP. There's a LNC skill for it but that obviously limits the accessability.
What kinda sucks about this is THM/BLM who basically don't have the issue because they have infinite mana via their main mechanic. It's fairly obvious how that's gonna be a problem eventually unless like one skill from a dow class does a lot more than a spell from a blm, as you'll end up having every DPS being a BLM for long fights as everyone else will run out of TP and do shit dmg. Also bard songs are shit apparently but that's a separate issue and requiring bards isn't good design either.
Maybe TP regen should increase over time if you don't use any, so like first tick is whatever it's at now, I wanna say 100, but if you don't use it 2nd tick is 120, then 150, then 200. So if you take a break from using shit you get a full bar of TP to play with again, or you can keep spamming like currently if you need to do the damage right now but lose damage over time.
So yeah I don't know about their system for now, it seems a bit shaky. I'm not too annoyed by the GCD though, but I guess I only played my MRD to 12 so I haven't played with the GCD that much(rest was casters and since spells tend to take as long to cast it doesn't really make a difference).