If you want a campaign where "it's DANGEROUS" to get into fights, D&D is the wrong game to be playing. D&D is about heroic warriors and battle mages--it's the role-playing equivalent of an action movie. Sure, Arnold gets his emotionally significant moments in True Lies, but we watch the movie to see him blow away the opposition with increasingly bigger weapons. Braveheart has the romance subplot and Robert the Bruce's character development but it wouldn't be much of a movie (it certainly wouldn't be the same movie) without massive medieval battle scenes. Aliens has its intrigue but we watch the movie to see the marines shooting and getting torn apart and Ripley strap into the power loader and kill the alien queen. It's the same in D&D. A good game has a lot of things other than combat but it has quite a bit of combat too.
The vast majority of the rules are devoted to combat so if combat isn't supposed to be a focus in your game, you're better off moving to a system where combat is less detailed and other situations have more detailed rules.
So that's why you need to have armor substitutes. Because the combat system doesn't work well if you don't compensate for lack of armor, if you don't come up with a substitute, you are ruining the majority of D&D's rules and balance. In fact, even the balance of classes and fighting styles requires appropriate equipment--in a low equipment game, characters will gravitate towards two handed weapons, barbarian rage, and power attack because without the option of decreasing the amount of damage you take through AC, increasing your damage output as high as it can go is the only way to win a battle. (The prevalence of Barbarian 2/Fighter 4 characters in early Living Greyhawk is a testament to this dynamic).