Car crashes are the leading cause of both disability and death for children under 12 years of age.
State laws provide children with very minimal protection for children in car crashes. They are absolute minimum guidelines that are only meant to guide parents on keeping kids safe in a cars.
Most states (like North Carolina) are very vague on the difference between a 5 point harness restraint and a booster seat. Some states do not even have minimum guidelines on when kids can be placed in a booster seat. Yes, a booster seat is a child restraint device.
How old is your child? What does your child weigh? (if you edit your answer, if I come back, which is highly likely, I will help you better and give you advice that will apply to you).
A child can safely transition to a booster seat when ALL of the following apply:
-the child is at LEAST 4 years old (preferably age 5-6 to get hip bones more developed)
-the child is at LEAST 40 lbs
-the child is capable of sitting still without playing with the seat belt for every single ride no matter how short (both awake and asleep)
The National Highway Traffic Safety Administration (NHTSA) recommends that kids ride in a 5 point harness car seat up until they are 8 years old (if they fit in one); this is because 5 point harness car seats provide children with much better protection in car crashes than booster seats. Most kids outgrow their harnessed car seats by height before weight- when the harness is at the child's shoulder in the top slot (the harness MUST be positioned at or slightly above the shoulders when front facing). The Evenflo Maestro has 18" top harness slots and a 50 lb weight limit. The Graco Nautilus has 18" top slots and a 65 lb weight limit. The Britax Frontier has 20" top slots and an 85 lb weight limit. The Evenflo Secure Kid 300 (or 400 model) also has 18" top harness slots and a 65 lb weight limit.
A booster seat's job is to boost the child up so that the lap belt is positioned low on the child's hips and the shoulder belt between the child's shoulder and neck.
The shoulder portion of the seat belt is not dangerous when it rubs against a child's neck. It is only dangerous when a child becomes uncomfortable and places it behind their back or under their arm. The consequences of doing so are shown in the video below at 4:05: http://www.youtube.com/watch?v=ULJ8Vx79Vv4
The lap portion of the seat belt is very dangerous when it is incorrectly positioned on a child's body. If the lap belt is on the child's stomach, the child will suffer from severe internal damage to their inner organs. Not all booster seats correct seat belt fit. You need to find the booster seat that properly positions the seat belt. Here is what seat belt fit should look like: http://www.iihs.org/research/topics/boosters/default.html
A backless booster is fine if the child has a headrest in that seating position that is (at the very least) up to eye level for whiplash protection. Some backless boosters come with a clip that positions the shoulder belt. Here shoulder belt clip is spoken (and used) at about 2:25 in the following video: http://www.youtube.com/watch?v=KNXHC_BoCN8
I would not use a backless booster if the shoulder belt does not fall naturally on the child's shoulder as most parents don't bother using the positioning clips.
There is nothing magical at age 8 or when a child is 80 lbs for that matter. The law came out when boosters only went to 80 lbs. Nowadays, boosters go up to 120 lbs. A child isn't suddenly safer when they are 8 years old than when they were 7. Most 8 year olds are not ready to use a seat belt alone in the car. The only accurate way to evaluate when a child is ready to use a seat belt alone is when the child passes the 5 step test. This test is a series of questions you ask yourself when the child is buckled in the car without their booster seat. The 5 step test is described in the following link (the articles were written by a child passenger safety technician): http://carseatblog.com/3966/the-5-step-test/
EDIT: I added this article because I think that it is very nice: http://carseatblog.com/17683/think-your-big-kids-dont-need-a-booster-think-again/http://www.boosttil8.org/
In the above link, Kyle who was 8 years old was in a car crash; he is know paralyzed from the neck down because he did not fit the adult seat belt. Something so simple as using a booster seat could have prevented his terrible injury.