The fact that sword and shield are separate items AT ALL is a remnant from 1.0 days. I'm sure that, had they not felt obligated to carry equipment over from that game, Sword/shield would have been a single item, just as NIN's two daggers are a single item, MCH's gun and Aetherpack are a single item, BRD's bow and quiver are a single item, Astrologian's star globe and card rack are a single item, and so on. There are a lot of awkward things in this game that result from sword and shield being separate items, some working in PLD's favor (for example, being able to mix and match shield and sword for better stats) and some working decidedly against (for example, PLD used to need TWO Mirrors of the Whorl to upgrade both sword and shield, which was patently unfair given how rare they were - not to mention the difficult of getting both the base sword and the base shield to begin with).

Additionally, if not for those 1.0 remnants, I'm guessing THM and CNJ would never have been able to equip shields, either. GLD/PLD is the only DoW/DoM that continues to make use of the off-hand slot, and given the awkwardness of it it wouldn't surprise me if, sometime in the future, the Devs finally get fed up and go ahead and merge them - every single set in the game - into a single item. It'd be a lot of work, but one that would likely pay off in the long run.