Originally Posted by
Enkidoh
It's also a legacy of FFXIV's sordid roots - as ARR is made on the bones of version 1.0, it had to accomodate how 1.0 was designed - in this case, originally in 1.0 shield use was a seperate class called 'Sentinel' or 'SEN', which had to piggyback on another class - specifically GLA, but CNJ and THM could also equip it, this is incidentally also the reason why CNJ and THM have some low level shields to this day available. When classes were redesigned to allow Jobs to be added to them, SEN was scrapped as a class, it's abilities rolled into GLA and it's new PLD Job, and shield use just became a 'shield' that still went in the offhand slot, just like the secondary tools of Disciples of the Hand and Land.
Due to 1.0 players being able to import their 1.0 character data into ARR, classes had to therefore be compatiable with the old data in the new game, and thus shield use continues to be an optional piece of equipment for GLA/PLD (this is also why GLA/PLD does not start with a shield but just a sword). It's clear SE are not going to change this.
I suspect the original poster's idea actually comes from a misinformed belief that rolling shield use into sword would somehow then increase inventory space, but as others have pointed out, the offhand slot is it's own independent section in the Armoury Chest and it's removal wouldn't freeup space like removing Belts did.