I have used one for my dd since she was about 4. The idea is just to get them the child the right height for the seatbelt to work properly. I feel comfortable using it, but then when I was little my dad's car didn't even have seatbelts!!!
Yeah I guess the booster cushion type thing makes sense for short trips (and slow speeds, in London traffic). Will see what DH thinks (medic - seen a lot of crash victims in his time so errs on the side of caution - if he thinks it's ok I'll be happy).