I don't think I would trust a backless in most cases..again depends on the existing back seat of your auto.
My main concern in a wreck would be immobilizing the head and neck area.
The only bennefit I would see in a backless would be ease of use, meaning your child would be more likely to use it..But with the car I drove when my Son was that age, he was safer just using my existing (extremely small) backseat with the seatbelt.
Yes, I am thinking a backed one would be better, thanks for all the advice on this thread, I just ordered one from Amazon.