Without proficiencies everyone would just wield the best weapon in a category. There would be no more variety for weapons and armor. All simple weapons would see no use at all, and everyone would wander around with a great sword, a longbow, and a longsword/large steel shield. It would be terrible.
I'm sorry, but I don't like the idea of a mage wiping out an entire party of PCs/Monsters because the wizard rolled a twenty on their sleep or glitterdust checks. A save from each subject is what balances the AOE disabling spells.