You are invoking the troll in me! 🙂
Noby, how were we driving before the power steering, the abs, the traction, the stability control, the cruise control, the breaking assist, the active suspension, etc.... They all remove the driver intuition (all of my trackday cars, and my first 3 street cars, have been and will be with none of those), yet I can promise you that without half of those you will be quite unhappy. ACC is no different - let me explain, because you obviously have not had one of those ...
When you are driving on the motorway you have 3 options:
1. pedals, in which case your concentration goes to that and your right foot is on the throttle. Unless you drive with both legs, which I highly doubt, in case of an emergency you need about half a second at best to react and position your foot on the break pedal, and then because your foot is not there and you are rushing for it, you usually overdo it and that on its own, with a high probability can result in the rear car ramming you, not to say what happens if you are actually not on a straight at that point.
2. you drive with CC, in which case your foot is on (or near) the break most of the time, but your concentration is divided between the pedals, the wheel and the buttons - to match the speed the car in front (that is one control more than needed and especially with the behavior of the trucks here, leads to unhealthy annoyance on a long trip). Then when that car in front accelerates faster than you can click buttons, your annoyance moves your foot off the break pedal to press the throttle ... this is when bad things happen.
3. you are driving with ACC, which keeps its distance from the car in front much better than you and compensates for speed changes reasonably. In the mean time your foot is on the break and you can react much faster and more adequately than in the other two cases. You are not distracted by speed fluctuations and can concentrate on the important things.
Then come the next two features:
Is your concentration really better than that of the electronics, after driving the most annoying road possible for 5 hours? No it isn't, electronics will always be more reliable than human for simple tasks. I know about at least two people that have died or killed someone else because they fell asleep (even if just for a second) behind the wheel - one crashed in the car in front, with a bit too much speed difference and killed his passenger, the other just hit a truck and was smashed by the one behind ...
More importantly, the ACC in Lexus of that generation comes with collision avoidance breaking system, which although doing only up to 0.3G of breaking, can still reduce your speed enough in the right moment and give you time to press the breaks - been there, done that. Keep in mind, that on a motorway, you are traveling your breaking distance before you hit the breaks.
I am not speaking about trusting the ACC here, exactly the opposite - trust nothing in a car, but use the max available.