What is the deal with not using the cruise control

I've seen a lot of people in this sub who seem to think there is something wrong with using the cruise control while driving. I would love to hear rationale for not using it.

Some of the pro's are:
Helps prevent speeding
Prevents you from doing something obnoxious like speeding up when someone is passing.
Saves gas.
Allows you to focus more on traffic.

In the aviation industry, they have a concept called "pilot load" - and they use the flying equivalent of cruise control (AKA autopilot) to be able to pay more attention and not get overwhelmed with flying the plane. In my experience, the same thing can be said for cruise control (when you set speed with cruise, you don't need to look down at your speedometer as much).

Based on my casual observations, the fact that people routinely vary their speed widely tells me they're probably not using it - is there a good reason not to?
Obviously heavy traffic (especially if you don't have adaptive speed control) would be an exception.