There's really nothing wrong with harnessing as long as possible. The whole 'neck load' thing hasn't been shown to be a danger to real kids in real harnesses in real crashes (nor in dummies in crash testing, the Swedish data has never been released in English, as far as I know, I'm sure the other thread has it posted, if it has, but really, it's a claim that's not been substantiated in our crash tests or in real life). Harnessing may still be safer (really, how can 5 points of restraint NOT be safer in more types of crashes? What's magical about only 3 points, especially in side impacts? Nothing, it's a standard devised in the 60's and 'good enough' up till now, as long as airbags are hovering all around us to make up for their lack of optimal functioning
), we don't know yet, all we know is that boostering is 'safe enough' for kids over 4, statistically, based on crash information gathered at crash scenes by EMT's who don't generally distinguish restraint type. So really, your choice. Honestly, it's been worth it to me to harness as long as possible for my own sanity (even my teenagers have to be reminded to sit still, why start that annoyance any younger than I have to?
), but for short trips and travel, I use backless boosters as young as 4 <shrug>