Playing with adaptive cruise's following distance can keep a driver engaged with driving situations while retaining the benefits of adaptive cruise. Not sure if there is an analogy for lanekeeping.

That is a problem even without this kind of technology.

This is a good point, although I do think there is an argument for a greater tendency to "mentally wander" if you aren't physically engaged somehow.

If you are long distance driving on straight empty highways, it is easy to get into the wandering state. I find these features only useful in those cases where engagement is difficult to maintain anyways.

Very true, and that situation is the only time the feature appears to work. Minimal repetitious physical engagement wouldn't change much I'm guessing.

Ya, I never use them for city driving. It is just that once a year cross country western trip where it comes in really handy.