Well, when DS was 4 and I returned to work, my in-laws and DH needed to get three across. We could do it with booster/KeyFit/booster but not with any other seats I owned. No combo of Marathon/Boulevard/Regent/ETFA/KeyFit/Booster was going to work. I could not buy two Radian's for one or two trips a week max. So, DS went into a booster.
He is still harnessed in my car, he'll be five in January, but in others cars, he is typically in a booster. He sits fine for short rides, I have no qualms about it, he moves to and from the harness in my car without complaint. He'll likely remain harnessed in my car to 5.5 or 6, and then be mostly boostered, except for 2+ hour drives.
DD will be 8 in February and still fits in a HBB by height, but she is LEGGY. Her legs ache after long rides in any of our HBB because she sits further forward. In a LBB she can scoot further back and her knees actually bend at the edge. She is more comfortable. However, she likes having the HBB to rest her head when she sleeps. She started riding occasionally in a LBB this fall, at 7.5 There was simply no reason to do so before.
If I had a "rule" it would be 4 and 40lb for HBB booster use, max height of high back for use of low back, but it is all dependent on the child, their parents and their vehicle.