The idea is that the harnessed seat restrains the child's torso so well, that the head can go flinging forward and cause neck injuries. In a booster, the child's body would move more, which would reduce the strain on the neck. This is a known issue for race-car drivers, who have tethers on their helmets to restrain their heads in an accident. It hasn't been well studied enough for my taste in children & automobile accidents, but I personally lean in the direction of boosters for kids over 5.
In Sweeden, which has a really good track record for child passenger safety, they don't harness FF children at all - but they also RF to 4-6 years old.
My personal plan is to RF DD until she is 40lbs, then harness her FF until she is 5 or so, then start booster training. She is fairly mature, so I think she will be ready to sit in a booster by 5.