Can't say I agree, use it all the time! In fact nothing infuriates me more than people who clearly have cruise control (judging by age/model of car) but don't use it! Bloody hate it when you keep overtaking then getting overtaken by the same retard for miles, I know I'm going the same speed what the hell are you doing!?
Might not be about them not using cruise control - some people seem to get irrationally agitated if someone overtakes them and will speed up and overtake and then slow down again, repeat. It gets quite silly sometimes.