well that was the idea back when they would designate a left ski and a right ski, but times they are a changing.
The thing is, say you're some weekend warrior gomer motherfucker and one of your old ass rear entry boots has a crack in the toe or something, but you don't bring that one into the shop to get your skis mounted with. No, you bring the one that isn't fucked up. So of course the boot/binding system tests fine with that boot. So yay your bindings are working perfectly right? But then you fall and the part of the boot that was cracked breaks off halfway and gets stuck in the binding, and therefore YOU are stuck in the binding, and since you have weak little girl knees you tear some ligaments. Sounds far fetched, but it happens.