This page always offers me some reassurance about harnessing longer...
http://www.tc.gc.ca/eng/roadsafety/safedrivers-childsafety-programs-testing-harness-report-923.htm
"When this study began in 2003, Canadian regulations set the upper weight limit for forward-facing car seats at 22 kg or 48 lb. Once children reached this limit, they would then move from their car seat into a booster seat. Although moving from a car seat to a booster can be exciting for the growing child and the family, it may offer less protection for some children.
Why? A children's car seat uses a 5-point harness system that is built into its shell. The harness straps are narrower than a car's seat belt and fit a child's small shoulders very well. In case of a sudden stop or crash, they will hold the child back and spread the forces of the crash over the two shoulders, the chest, part of the belly and the hips. This way, the child's body can come to a controlled stop with less chance of injury.
A booster seat, on the other hand, relies on the car's lap and shoulder belt to protect a child. Instead of a harness system, it has guides that help keep the car's seat belt on the child's hips and on one shoulder. Keeping the belt on the shoulder of a small child can be difficult, especially during a crash, because a child's shoulder is much smaller and more rounded than an adult's. And even if the booster seat does a great job of keeping the belt in place during a crash, it still may not hold back a child as safely as a children's car seat would.
In a study on booster seats, we found that in many cars, the booster seats were not able to spread the forces of a crash or control the upper body or chest of the child dummy. This led us to ask whether children could be better protected if they stayed in children's car seats for as long as possible."
As usual, 'as long as possible' is not defined, but given these tests, I have no worries at all about keeping kids harnessed to at least age six (or 7 or 8, frankly, harnesses and belts stretch like crazy, that reduces a lot of force on the neck, a carseat installed is absolutely a different animal from a harness bolted directly to the car frame...they have found those to be very dangerous not only in NASCAR, but in Australia, where harnesses added to boosters have been fairly popular for young/heavy kids, and now they are not recommended).