I would (and did) but for a completely different reason....
They can hold onto their things better
.
We just did 6 hours in the car, and the only one who was happy the whole time was the 4 year old, RFing in her BLVD70. She had her My Little Ponies in her lap, her water cup, Goldfish, and frozen soybeans tucked into the seat next to her, and her pillow and blanket. Nothing fell, nothing was out of her reach. It was great!
The boys, on the other hand, were total pains, and not on purpose. They are 6 1/2 and 7 1/2, and both in boosters. I was constantly reminding them to sit up, not to grab things off the floor, handing things back to them, etc. If they were in harnesses I would have flipped them RFing too