I can only say that removing the GCD on caster would make things funky for red mages who have an instant cast after every hard cast. As it is now, a red mage has a small window to move between casts, but if the GCD were reduced to just the cast time, then they go straight from cast to dualcast to immediately into another cast.

This also would effect a job like black mage where it may become more effecient, which good button mashing skills and high ping, to just spam scathe rather than do the rotation, and would make summoner's Ruin 2/4 have a similar outcome.