Yes. Think about it this way: if having Autopilot on requires the same amount of attention/effort as having it off, then there would essentially be no point to the system whatsoever. The entire idea is that the system takes over some of the driving tasks to make the experience more pleasant, just like how regular cruise control is convenient because it frees you from having to pay attention to maintaining the car's speed by pressing the accelerator pedal.

The question is, does it reduce the amount of attention paid to a significant enough degree that it's dangerous? Consider subways -- most of them have automatic train control systems in place that could effectively drive the trains automatically. However, operators are still required to control the speed, largely because it keeps them engaged.

Give an operator nothing to do, and they won't pay attention -- this is basic human psychology (look up "Vigilance Tasks" if you're interested in further reading).