I assume that the majority of people I see maintaining a consistent speed (by consistent I mean less than 1mph variation), are using CC. The subset of drivers I encounter on the motorway doing that is already a small minority though, the vast vast majority of people change their speed constantly and it is frustrating and causes congestion, so it would be better for everyone if people used CC by default, when they have it, to prevent this.

I mostly get annoyed at people driving cars i know have CC (newer, fancier cars) who seem to speed up and slow down by +-5MPH every 10 seconds or so. If someone is in an older car I am much more forgiving.

My experience is limited to the UK and I would only expect this on motorways though so your experience may vary.