Getting distracted with Autopilot engaged is like removing your seatbelt because you have airbags.
I would argue that getting distracted with Autopilot engaged is a virtual certainty.
Unlike cruise control, the system completely takes control of the car from the driver. The driver is left with nothing to do but look at the scenery.
I think it would be incredibly difficult to remain completely alert, yet relaxed in this situation. I, personally, would either:
1. not trust the system and would be constantly stressed out that it was trying to kill me, or
2. would get overly comfortable, distract myself for increasing amounts of time, and then accidentally fall asleep or completely focus on a different task, like reading.
Realistically, neither of these options work for me. So I would not use autopilot unless it was functional at a level where it could actually fully drive the car and I was willing to cede it complete responsibility for my life.
Now I acknowledge that people are different and that some will be able to continuously stare at the road with complete attention while not driving. My wife for example is really good at this...