Somatic spell components require a free hand, but usability is not very good in that UI flow (->open inventory, unequip shield/off hand weapon, cast spell, next turn: reverse order).
You could argue that weapon configuration slots are for that, but that doesn't work well. Usually one is for melee, and one is for ranged. The third customization option is for light.
-A button/shortcut to equip/unequip off-hand (game needs to remember the item that was unequipped)
-Or a 4th configuration slot, similar to the light preset, that is re-using the currently held 1h item. Specifically designed for "free off hand" config.