my almost 6 year old is still in a 5 point harness. In an pinch, I'll let her ride with just a booster, backless or otherwise, but not for every day or as a routine. My boys (at 7) were in a backless booster in one car, but have their regular boosters, with backs, for in the van.
I was under the impression that the backed booster was for fit, not because one was safer than the other, assuming that the child fits in both. So, if, at 6, your child fits properly with just a booster, I don't think there's improved safety between the backless and the regular booster. There is improved safety with a 5 point for as long as it fits. That's my understanding, anyway.