I'm totally prepared to be put right here as I can only go on my own experience!
DD is 7, and is over 10cms too short to go without a booster seat - we don't have a car but she rides in a high-backed booster which her car seat modified into, we have a Boostapak with a shoulder strap clip which she uses in friends cars/for lifts etc when the high-back doesn't fit - on a booster on its own she is a little short for the seatbelt to sit over her shoulder.
Most of her friends sit just on booster cushions, including those who are much younger, a lot of mums seem to chuck the high-backed booster in favour of a cushion as soon as their child hits the minimum height/weight, a couple (who's DC's are the same age/height/weight as DD) have got rid of boosters completely, and are bemused and roll eyes at me when I pop her seat in the car. One even encouraged their DC to call my DD a 'baby' for still using a 'baby' seat. DD, for the record, couldn't care less - she likes her seat, it's comfy and she knows why children need seats/seat belts etc - on this occasion she quipped back "at least I won't die if we crash"....... (I kinda wanted to tell her off and kinda didn't lol)
All of DD's friends parents don't need to transfer their seats out of cars or store them out of cars, so that's not an inconvenience. Am I totally off the ball with this? I've always kept DD in a seat until she hits the maximum weight rather than the minimum - it's not hard to do - and worth it if she is safe, but moving up from car seats seems to have become the new 'walking, talking and potty trained' race from toddler-hood! Am I missing something? It's not a race, it's about safety!?!?!