This seems like... An obvious conclusion? Also since no level 3 or level 4 cars exist (besides the Mercedes in extremely limited applications), this essentially is untestable in the real world. How the car responds to a disengagement seems to be the key issue. Apparently the Mercedes gives you 10 whole seconds to respond... Which should be more than enough time to pay attention and put down your phone you're texting on.

I can say from experience, that TACC and lane keeping significantly reduces my mental load while driving, especially for long periods of time. Highway driving constantly requires you to center the car, and something that does that for you makes doing the rest of the driving (watching out for traffic issues, construction management, etc) much much easier in my view. That's only level 2, however. It still requires you to pay attention, it's just a different kind of attention in my view.