Having jaunted up to Dallas-Fort Worth and now back for Thanksgiving, I find myself more frustrated than ever at the number of drivers that do not use cruise control. I get frustrated by the ones than drive 1-2mph faster than me, then 1-2 mph slower, forcing me to up my cruise 1-2 mph to pass them and stay past them. (This ignores those, whether out of Texas macho, or more generic macho, who then insist on passing me back again.)
On freeways outside of urban areas, left-lane lopers are the only species of bad driver, I think, that frustrates me more. And, if you need to have THAT phrase explained to you, you need to have your license revoked.
Back to the non-cruise users.
First, you're disrupting traffic flow. (You can step on your gas pedal to pass more quickly, whether on a freeway or a two-lane road, then go back to cruise control speed after completing your pass, you know.)
Second, using cruise control is probably the third most effective gas mileage enhancer a driver can do, after proper tire inflation and no jackrabbit starts/stops. Given modern engines, it surely ranks ahead of tune-ups, which aren't regularly needed anymore.
Cruise control technology continually improves. Even 15 years ago, automakers had reduced a lot of the "hunting" that cruise controls do with automatics, and how readily they do or do not downshift. Today, with more and more new cars coming with five-speed automatics, if and when they do downshift, it's more smoothly than ever before.
So, let's get with the program, folks.