There is a family in TX that has been getting a lot of media attention since they were in a tragic car accident. Mom and Dad were killed, 8 and 9 year old boys were paralyzed, but their 6 year old sister suffered minor injuries, some broken bones. In TX, children 8 and over do not have to be in a child restraint.
I am guessing that Willa was in a car seat or booster and her brothers were not, because this article describes the extent of the boys injuries. "Both boys have multiple spinal injuries from their seat belts and had pins inserted to stabilize the spine. Peter has a lot of damage to his lumbar spine. Both boys have contusions to the thoracic spine. Aaron had a halo installed to treat whiplash."
I am trying to learn from this tragedy and continue to keep my kids in 5 point harnesses. I went out of town a few weeks a go with a friend and the kids used boosters so everyone would fit in the back row of the SUV. On the way home, all the kids were asleep and leaning forward. It was not safe at all, but I felt better knowing they would go back to their 5 pt harnesses. My friend's kids use a high back and backless boosters all the time.
My 8.5 year old and 5 year olds are in the Britax Regent. My 3 year old is RF in a Britax Roundabout.
Obviously we can not control everything in life, but I feel like this is something that I can continue to do, to keep my kids safe.
This link describes injuries sustained by Seat Belt Syndrome
Boosters protect against seatbelt syndrome. There's nothing wrong with harnessing older/larger kids, but there's also nothing (and many of us techs/advocates have looked long and hard for any such statistics) showing that harnesses are safer than PROPERLY USED boosters. Proper use, of course, incorporates developmental and behavioral readiness. And lap-only belts are never safe with or without boosters.
Properly used boosters are safe: there is no need to warn against them.
Chickabiddy is quite correct. Your 8 year old is safe in her Regent, presuming she meets the height and weight minimums for it (the straps are routed through the nearest slot at or over her shoulders, not under, she's under 80 lbs, and her shoulders don't go over the top slot, and the tops of her ears don't go over the top of the seat), and the seat is installed correctly (recline bar used according to direction, top tether always in use).
That being said, your daughter is not safe-ER in her Regent than she would be in a properly fitted belt positioning booster. Belt positioning boosters protect against seatbelt syndrome.
The injuries the children received as described above are not only consistent with being out of a booster too soon, they are also consistent with something kids tend to do when the seatbelt doesn't fit correctly: they tuck the shoulder belt behind their backs or under the arms so they're more comfortable. This creates a 'lap-belt only' phenomenon in a crash, which leads to the 'seatbelt syndrome' you described above.
High back and backless boosters are very, very safe.
Part of training children to use them correctly is teaching them how to tip their heads back when they nap, so they don't slump forward in the belt.