Maybe it would help if we could delineate all the common wield scenarios. I'll give it a go. Tell me if I missed anything.

1. Dual-wielding, want to change both hands.

2. Dual-wielding, want to change main hand w/ another one-handed weapon.

3. Dual-wielding, want to change off hand w/ another one-handed weapon.

4. Dual-wielding, want to wield a single two-handed weapon.

5. Wielding a weapon and a held item, want to swap weapon for another one-handed weapon.

6. Wielding a weapon and held item, want to swap held item for a one-handed weapon.

7. Wielding a weapon and held item, want to swap both for a single two-handed weapon.

8. Main hand is empty (disarmed) and off-hand has weapon, want to wield a one-handed weapon in main.

9. Main hand is empty (disarmed) and off-hand has held item, want to wield a one-handed weapon in main.

For each of these there are three special cases, all of which could be simultaneously in effect:

A: Affected by kotegaeshi (i.e. only one wield/wear spot).

B: The desired action is not possible due to weight restrictions (i.e. you couldn't wield the weapon even if both hands were empty because you're just too weak)

C: The desired action is not possible due to dual-wield restrictions (i.e. you're trying to dual-wield something that's too light/heavy, or you'er trying to main-wield something that's too light/heavy for your dual wield).

(2) and (8) are where I could see the "automatic" behavior haveing undesirable effects, since it could result in your dual-wield being removed in order to complete your main-hand wield request.

(3) could also be weird. What do you want to happen if you try to swap your dual-wield and the weapon being swapped in is too heavy/light to be dual-wielded with whatever's in your main hand? Should the main-wield be removed and the "new" dual-wield be placed in the main hand? That seems wacky.