Now, completely ignoring the whole part about "Limited storage space AND they destroy your items" being a terrible idea for a moment.
Here's how they should work when applying them to your gear.
Equip Plate 1 while PLD, save gear.
Equip Plate 2 while DRK, save gear.
Switching to PLD displays Plate 1
Switching back to DRK displays Plate 2
Now you can be Lightning while a PLD and Cloud when a DRK and you can do it on the fly, while sharing the same gear between the two. Go fanboy it up.
Simple. Easy. Convenient. Makes sense and is not annoying.
Here's how they actually work.
Equip Plate 1 While PLD, save gear.
Equip Plate 2 while DRK, save gear.
Switch to PLD, and you're still wearing Plate 2. Realize that they are literally just overwriting your glamour on the same shared gear over and over, completely defeating the point of any of this. Go back to town every time you want your PLD to look like Lightning and your DRK to look like Cloud. You loser.
This is bad and lame.
A glamour plate should be a completely separate entity that shouldn't project over the gear you have equipped like a normal glamour. It should become a single "Item" with it's own unique slot on the paper doll that simply ignores all of your gear in favor of the projected appearance of the plate.