With the total lack of EXP gained doing instances there will be little to no replay value for them. I like the fact that they are requirements to progress the story, but what we will start to see as time goes by is no one queuing for anything they have already done. New players will hit walls every time they need to complete an instance.

Players level up extremely fast. Gear all the way up to level 50 is transitional gear. There is no point in repeating instances to get the items you want, nor is there a real point to adding materia to gear before level 50. This is yet another reason players will not be repeating instances.