Race car drivers wear a HANS (Head And Neck Support) device, which is what tam was referring to, I think. It is an extra piece of equipment that goes around the neck and shoulders and the driver's helmet tethers to it on both sides of the head. It prevents the head from whipping forward in relation to the body which is held back by the harness. As tarynsmum mentioned there is concern in some circles with the body being held back too firmly in a harness while the head whips forward - the whole reason why rear-facing is safer. I have also heard concerns about high loads in the groin area.
So, the race-car driver comparison is moot, since they have extra equipment that mitigates the high neck loads. I'm not sure a vehicle seat belt is any more or less likely to fail than a restraint harness or a UAS anchor/connector, so that wouldn't enter into my thinking at all. Plus for a heavier kid you may have to install the restraint with the seat belt anyway (UAS anchors have weight limits - most are either 40 lbs or 48 lbs), so you'd still have that as an issue.
As previously stated, there is no good evidence on this either way. It actually kind of bothers me that the common institutional line is that a step up to the next type of car seat is a step down in safety, because we only have evidence that this is actually true in the case of RFing to FFing. The only thing that lets me live with that statement is the presumption that it is likely directed at the hoards of people that put their 3-year olds in boosters, which is clearly not safe. Switching to a booster is ONLY safe if your child is at least 40 lbs, and is mature enough, which usually means at least 5-years old, maybe older. It also matters that the booster fits properly. I'm not sure which is most prevalent - incorrectly installed harnessed seats, or improperly fitting boosters. Both are sadly common.
So, all that said (that was way longer than I intended!), it is a parental decision. If you go booster, make sure the child is ready and the booster seat makes the seat belt fit properly in every car it will be used in. And know that correctly used booster seats are perfectly safe. I'll be trying my 5-year old out in a booster in the fall once he is solidly over 40 lbs (still varies right now) and our summer road-tripping is over. The potential increased neck-loads issue combined with the fact that correctly used boosters are safe puts me in the camp of boostering when appropriate (i.e. not staying in a harness if there is no need to do so). But that's just me.
ETA: In case you are curious about the HANS device:
http://en.wikipedia.org/wiki/HANS_device