IRL I was taught to hunt small game with throwing knives as a kid. The effective kill range is basically laughable and I probably would have better luck throwing a rock. Tomahawks and hatchets are a different matter and weren’t available in my age bracket when I did competitions, but according to my mother they’re definitely something with a lot more killing power, not that anyone would actually hunt with them naturally.
But wielding them makes perfect sense to me. The only thing I ever held in my off hand while throwing was more knives. Given that you can wield a stack of them this is already modeled in game, but I don’t think anyone could reliably hit anything by throwing with their off hand or holding something that would effect their balance considerably. Honestly torso encumbrance should penalize throwing too based on my experience.
That being said you would still need to wield the weapon, as retrieving it from a backpack isn’t instant and really shouldn’t be. You can drop or stow what you were already holding, but throwing weapons with any force really is a full body action. Look at a baseball player throw a fastball and you’ll get an idea of the mechanics involved.