Armoury chest – could offhand slots be redistributed?

[Jojoya]

If 15 is too tight... I'm not sure what the alternative would be. But you don't tend to amass offhand items in the same way as other categories, since tools don't drop from dungeons and there's no point to keeping old ones after you replace them. Any amount of fiddliness it would add to managing your offhand slots would probably be well made up by less fiddling with the other categories.

PLD can actually amass a large number of shields to go with different glamours. Crafters who level up their DoH classes all at the same time would also quickly hit the 15 limit as they work to upgrade their gear. Gatherers have the same issue plus the need to hold extra for their retainers if they're using DoL retainers.

Granted the need tends to be temporary but it does occur. That would mean needing at least 25 slots for offhand at times.

I don't think there is a good solution. Some players are making use of those slots because they use classes/jobs with offhands. Others don't.