The extra mag isn't usually for fear of running out of ammo. It's because the most likely cause of a malfunction is mag related so the spare is to use if the first one fails for any reason. And for many of us the knife is more of a task oriented item then a defensive weapon though in a pinch it can serve that role as well.
Your point has validity. With that said, I put 500 rds through a single mag with the pistol before even considering it reliable. I have the utmost confidence in my carry pistol and mag.
As another posted, in a civilian self defense situation, the ammo in your mag and pistol are your primary for the defense and the fight duration fairly short. Law enforcement however, is a different category all together. Changing mag during a firefight gives the attacker time to return fire.