I thought of this when thinking about a situation where there is an enemy in very close proximity and your primary weapon is out or close to be out of bullets in your magazine. Instead of switching to the secondary or reloading, you should be able to simply drop your weapon and pull out your handgun for quick retaliation.
Furthermore, every primary weapon should have a sling around you so when you do drop your primary it will just be hanging off you, not on the ground.
Making a sling an equipment feature would be an awesome idea to allow this. If running an unslung rifle, you could make it so that only one hand draws and fires the secondary while the other guides down and retains the rifle. This would do well bringing more real shooting techniques to the game.
great idea, i think it can go further than just for fast switching, it could allow to temporarily switch out primary weapons without having to drop your current weapon and magazines, and then switch back again after use.
a simple example would be sneaking up on a sniper, take him out, and only be able to pickup the enemies rifle with current magazine, thus temporary use. Once you switch to either primary rifle or sidearm, you will drop the weapon on the ground.
Think other situations where a specific weapon could be very useful at a particular moment, and without the need of using the inventory, you can just quickly pickup an enemies primary, be it a rifle for sniping, support gunner, RPG, etc.
This way you do not need to go into an inventory to manage your gear in critical moments where the inventory could just be what gets you killed.