Being exhausted causes you to take foolish risks, perhaps ruinous sorts of things. If you twist an ankle or bang up your knee, post shtf, it will be all over for you but the crying. Exhaustion makes it harder for you to notice things, the sort of mistake that gets people killed. Like your not noticing the tripwire, or other signs of enemy presence or not noticing a food source, a source of natural danger, such as avalange, mud slide, lightning strike, etc. Fatigue lowers your body's resistance to disease, too. So, anything that increases your fatigue, like more weight in your BOB, had better be something that you don't DARE be without.