I see two main problems with having spells stationary casts only:

1. This game is supposed to be more fast paced and roaming, so if melee jobs are always on the move then mage jobs need to be able to keep up with them. Melee would constantly just move out of the range of spells being cast.

2. In fights like ifrit, this would mean certain, constant death for mages. I don't know how often I die as a conjurer because ifrit makes an explosion right under my feet and I can't get out in time. This would make that 10x worse. Mages are frail characters, and need the mobility to keep themselves alive while trying to keep everyone else alive. The current system of mages having to stop moving when the spell is actually being cast (and not just conjured) is bad enough, although acceptable given its rationality.