TC doesn't just misunderstand BLM, he misunderstands the game mechanics in general.

Cast time is entire seperate from GCD, and they have their own functions.

Cast time is the amount of time it takes to charge up the spell, requiring you to remain stationary. You can have cast times of 1s, 2.6s, even as much as 8s. These implement 'cost' in the form of 'time remaining stationary'. This is time you are unable to react to mechanics.

GCD is your minumum time between weaponskills and spells. Sometimes it's consumed by the cast time, sometimes it's not.
You are free to move about during the GCD, generally.

The benefit of instant casts spells, is that you can utilise your GCD to move and react to mechanics, instead of sitting there like a muppet while you get hit by an AoE.
It is not the ability to chain cast high potency spells in rapid succession. That would be insanely overpowered.


This is stuff you should have learned before level 50.