My personal experience with my 9YO twins (yes, they still use boosters) is that with a backless booster, squirmy kids are more likely to wiggle around because they don't have the sides of the high back reminding them to keep their head and torso in position. Also, for many vehicles, the belt guides on the high back enable better placement of the shoulder belt on younger booster riders. And if your kids still fall asleep on long car rides, the high back helps them stay in position even when asleep.
That said, there are times when my kids would use the backless boosters - if they were carpooling to a playdate and for some field trips where they needed to carry the boosters themselves. But it wasn't really until this year that they started using the backless boosters on a daily basis (but they still use the high backs for long trips b/c it's more comfy for them esp. if they get sleepy)