DS2 is 5 years, 3 months. He is well within the range of his harness limits, and while he is pretty mature and I think I could trust him to stay upright and positioned in a booster, he still falls asleep regularly in the afternoon on a car ride, so he would not be a candidate yet for a booster. He's even lighter than his big brother was at that age, so I think he will go harnessed well after his 7th birthday too.
We've never been in a situation where it might be easier to use a booster for a one-off ride. I'd probably be comfortable with that for DS1, but not DS2 yet.
I do look forward to the day that the kids need no help getting in and out of the car, and air travel with three kids sounds downright simple if you take the carseats out of the equation, but that can wait...safety first!