Oh, my primary suggestion wasn't to have the secondary weapon fire from left hand. While that would be a nice feature, I am aware that it would mainly just complicate things. It was more of a third mouse button* allowing a quick switch between weapons, and allowing weapons to be stored in ways that do not involve moving to a specific order in the inventory.
Let's say this example.
[Brackets
] means that the weapon is selected in right hand.
(Parenthesis
) means the weapon is stored in left hand.
Sniper Rifle
Assault Rifle
Handgun
Knife
Grenade
Suppose the player wants Sniper Rifle next to Knife so that if a player gets close, they can use the knife, rather than scrolling having to scroll down to it just by using mouse3*.
[Sniper Rifle
]Assault Rifle
Handgun
(Knife
)Grenade
Or:
[Sniper Rifle
]Assault Rifle
(Handgun
)Knife
Grenade
Now suppose a player is entering close combat, and wants to switch to Assault Rifle:
Sniper Rifle
[Assault Rifle
]Handgun
(Knife
)Grenade
Suppose the player expends all their ammo and needs to reload and some hoodlum sneaks up on them in close quarters. Use mouse3 and they now have their knife out putting their Assault Rifle in their left hand, rather than frantically having to scroll to the knife.
If anything, this is a kind of "in-a-pinch" tool.
*Or any other keybind. This is my preference though.