What is the legitimate use of Tesla's self-driving function? You're not legally allowed to drive without being in control of your vehicle. Tesla says you're supposed to remain aware and in control. You aren't allowed to sit there eating a sandwich, or take a nap, or deal with the baby in the back seat. Yes, people drive distracted all the time, but encouraging them to do so isn't a good idea, and doesn't make it legal or remove personal liability. Any argument that people do these things anyway (well, not nap normally) is going to be offset by the fact that autopilot encourages and increases these behaviors.
I love the idea of self-driving vehicles, and when it's good enough, I'd trust it more than I'd trust most drivers (but of course not more than I'd trust myself, hypocrite that I am). We're not there yet, though, so I again ask what the legitimate use of it is.