I do think they could've culled out some of the off-hand space to distribute when they last shuffled things. We've definitely got more space in there than we'll ever need.

Frankly, I'd even propose the off-hand slot be next on the chopping block if space becomes tight. What would be lost? Crafters and gatherers both could have the secondary gathering simply made into a trait or something. I don't even think there's any visual difference at all in the MIN/BTN off-hands, which you only ever see when they're gathering at relevant spots. And obviously FSH only has the one which will forever feel dumb.
Crafters, again, only ever see it if they're crafting a relevant item. For them, yeah, there is some visual variation. But I don't think it would really be much of a loss. Tie it to main-hand or level or something if you want it changing and looking different periodically. But really, does anyone ever actually glamour over any crafter off-hands? I'm not even certain that's a possibility..

That just leaves Paladin shields that they'd probably have to design a dedicated glamour slot to. There'd be too much pushback if they forced everyone to always use a matched set of main/off-hand. But. Since they don't really have a precedent, probably no certain easy way to make shields purely glamour. Hard to say how well they could design such a system.

(technically also have to take into account ARR WHM/BLM using shields as well... but I don't really think that would add too much complication..)

In summation, PLD is probably the only reason we still have the off-hand slot taking up space. Kinda a shame, after all these years and all the expansions, they've never designed another new job that uses it.