GCD questions

[Sylvain]

Hi, so we've all seen some RDM video by now showcasing the fast space at which this job dps.
However, there's something that bothers me.

Basically, RDM cast time can be put into three categories

=GCD, 2.5sec cast, this include the melee skills, verfalre and holy
<GCD, 2sec cast, the quick Verskills, melee embued,
>GCD, 5sec cast, the "u need the quick cast" skills, Verthunder,Veraoe


The melee embued spell (when you have enough black and white mana), also reduce the GCD to 1.5.
But all instant or shorter-than-GCD spells have a recast time of 2.5.
So, let say I stand still and cast 3 times the same spell in a row, is there any difference between these 3 spells?
instant, recast 2.5, potency 300
2sec, recast 2.5, potency 300
2.5sec, recast 2.5, potency 300

If there is no effective difference, then what is the point of having a shorter casting time if you have to wait for the GCD to end anyway. (beside mobility)
I don't say that instant shouldn't have GCD, but what is the difference between a 2sec cast and a 2.5sec cast if you have to wait 0.5sec after the 2sec cast?

Same goes for the healers, does this mean that if I spam CureI, I effectively can't do nothing between everycast for 0.5sec?

I never paid too much attention as the only caster I played was BLM which doesnt have this problem (Since FIV has a longer cast).
But I'm considering RDM as an alt and this made me wonder about the nature of the GCD in this game.