Have you ever driven in an area that isn't flat? Constantly checking speed is taking your eyes off the road frequently. There's nothing wrong with cruise control and still requires attentiveness.
I am not one of those people who think that my way is the right way and must enforce it upon everyone else. That being said, to me, I never use cruise control. The only place it can be used reasonably is on a long and empty highway; otherwise, you will be constantly messing with it. With adaptive cruise control, you won't have to constantly mess it; however, there is a second issue: Your feet and mind need to be instantly available from time to time, and when using cruise control, I find that my feet and mind are not instantly available.
It is simpler to just always be in control. Maybe not easier from an energy usage perspective, but I don't have to deal with suddenly taking full control. Which, over time, is ultimately easier for me.